Molecular switches and methods for making and using the same

ABSTRACT

The invention provides molecular switches which couple external signals to functionality and to methods of making and using the same. The switches according to the invention can be used, for example, to regulate gene transcription, target drug delivery to specific cells, transport drugs intracellularly, control drug release, provide conditionally active proteins, perform metabolic engineering, and modulate cell signaling pathways. Libraries comprising the switches and expression vectors and host cells for expressing the switches are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/507,466 filed Sep. 10, 2004, which is a 35 U.S.C. 371 U.S. nationalentry of International Application PCT/US2003/007380, having aninternational filing date of Mar. 10, 2003, which claims the benefit ofU.S. Provisional Application No. 60/362,588, filed Mar. 11, 2002, thecontent of each of the aforementioned applications is hereinincorporated by reference in their entirety.

FIELD OF THE INVENTION

The invention relates to fusion molecules which function as molecularswitches and to methods for making and using the same.

BACKGROUND OF THE INVENTION

Gene fusion technology, the fusion of two or more genes into a singlegene, has been widely used as a tool in protein engineering,localization and purification. There are two conceptually differentmethods of making fusions. The simplest method, end-to-end fusions, hasbeen used almost exclusively. The second methods, insertional fusion,comprises the insertion of one gene into the middle of another gene.Insertions can result in a continuous domain being split into adiscontinuous domain.

One of the first reports of successful insertion of one protein, intoanother was a study by Ehrmann, et. al., Proc. Natl. Acad. Sci. USA 87:7574-8, who described the insertion of alkaline phosphatase (AP) intothe E. coli outer membrane protein MalF, as a tool for studying membranetopology. High levels of alkaline phosphatase activity were obtained inthe fusions despite the fact that alkaline phosphatase requiresdimerization for activity. Since then, AP has been successfully insertedinto a number of integral membrane proteins (see, e.g., Bibi and Beja,1994, J. Biol. Chem. 269: 19910-5; Cosgriff and Pittard, 1997, J.Bacteriol. 179: 3317-23; Lacatena, et al., 1994, Proc. Natl. Acad. Sci.USA 91: 10521-5; Pi and Pittard, 1996, J. Bacteriol. 178: 2650-5; Pigeonand Silver; 1994, Mol. Microbiol. 14; 871-81).

Other proteins, including green fluorescent protein GFP (Biondi, et al.,1998, Nucleic Acids Res. 26: 4946-4952; Kratz, et al., 1999, Proc. Natl.Acad. Sci. USA 96: Siegel and Isacoff, 1997, Neuron 19: 735-41; Siegeland Iscoff, 2000, Methods Enzymol. 327: 249-59), TEM1 β-lactamase(Betton, et al., 1997, Nat. Biotechnology 15: 1276-1279; Collinet, etal., 2000, J. Biol. Chem. 275: 17428-33; Ehrmann, et al., 1990, Proc.Natl. Acad. Sci. USA 87: 7574-8), thioredoxin (Lu, et al., 1995,Biotechnology (N Y) 13: 366-72); dihydrofolate reductase (Collinet, etal., 2000, J. Biol. Chem. 275: 17428-33); FKBP12 (Tucker and Fields,Nat. Biotechnol. 19:1042-6); estrogen receptor-α (Tucker and Fields,2000, supra), and β-xylanase (Ay, et al., 1998, Proc. Natl. Acad. Sci.USA 95: 6613-6618); have been successfully inserted info other proteins.Such fusions at least partially retain the function of the insertedprotein.

Doi, et al., 1999, FEBS Letters 453: 305-307, describe a fusion whichcomprises an insertion of the β-lactamase inhibiting protein (BLIP)polypeptide into a surface loop of the GFP protein. After several roundsof random mutagenesis, polypeptides were obtained which exhibitedincreased fluorescence upon bind of a ligand (β-lactamase) to the BLIPpolypeptide.

More recently, yeast sensors for ligand binding were constructed by theinsertion of FKBP12 and the estrogen, receptor-α lgand-binding domaininto a rationally chosen site in dihydrofolate reductase (DHFR) (see,e.g., Tucker and Fields, 2001, Nature Biotechnology 19: 1042-1046). Thesite of insertion was at residue 107, a site previously shown to be onetolerant of bisection (Pelletier, et al., 1998, Proc. Natl. Acad. Sci.USA 95:12141-12146). The two fragments of DHFR divided at 107 were foundto be unable to reassemble to form an active enzyme unless the fragmentswere fused to domains that dimerized (e.g., such as leucine zippers).Yeast expressing the FKBP12-DHFR or ERα-DHFR fusion proteins bad anapproximate two-fold increase in growth rate in the presence of theirrespective ligands (FK106 and estrogen) when DHFR activity limitedgrowth. The fusion proteins were either fortuitously temperaturesensitive (ERα-DHFR) or designed to be so by mutation (FKBPI2-DHFR) inorder that subtle changes in growth could be detected upon addition ofthe ligand.

Generally, methods for generating fusion molecules have not provided asystematic way to functionally couple protein domains.

SUMMARY OF THE INVENTION

The invention provides molecular switches which couple external signals,including, but not limited to, the presence, absence or level ofmolecules, ligands, metabolites, ions, and the like, the presence,absence, or level of chemical, optical or electrical conditions, tofunctionality. Preferably, the switches are fusion molecules comprisingan insertion sequence and an acceptor sequence for receiving theinsertion sequence, wherein the state of the insertion sequence iscoupled to the state of the acceptor sequence. For example, the activityof the insertion sequence can be coupled to the activity/state of theacceptor sequence.

The “state” of a molecule can comprise its ability or latent ability toemit or absorb light, its ability or latent ability to changeconformation, its ability or latent ability to bind to a ligand, tocatalyze a substrate, transfer electrons, and the like. Preferably,molecular switches according to the invention are multistable, i.e.,able to switch between at least two states. In one aspect, the fusionmolecule is bistable, i.e., a state is either “ON” or “OFF”, forexample, able to emit light or not, able to bind or not, able tocatalyze or not, able to transfer electrons or not, and so forth. Inanother aspect, the fusion molecule is able to switch between more thantwo states. For example, in response to a particular threshold stateexhibited by an insertion sequence or acceptor sequence, the respectiveother sequence of the fusion may exhibit a range of states (e.g., arange of binding activity, a range of enzyme catalysis, etc.). Thus,rather than switching from “ON” or “OFF”, the fusion molecule canexhibit a graded response to a stimulus. More generally, a molecularswitch is one which generates, a measurable change in state in responseto a signal.

In one aspect, a molecular switch can comprise a plurality of fusion,molecules responsive to a signal, which mediate a function in responseto a change in state of at least a portion of the molecule. As above,preferably, this change of state occurs in response to a change in stateof another portion of the molecule. While the states of individualfusion molecules in the population may be ON or OFF, the aggregatepopulation of molecules may not be able to mediate the function unless athreshold number of molecules switch states. Thus, the “state” of thepopulation of molecules may be somewhere in between ON or OFF, dependingon the number of molecules which have switched states. This provides anability to more precisely tone a molecular response to a signal byselecting for molecules which respond to a range of signals andmodifying the population of fusion molecules to provide selected numberseffusion molecules which respond to a narrow range or wider range ofsignal as desired.

In yet another aspect, the invention provides a fusion moleculecomprising an insertion sequence and an acceptor sequence. The insertionsequence or the acceptor sequence localizes the fusion moleculeintracellularly. Preferably, the fusion molecule is associated with abio-effective molecule and intracellular localization is coupled torelease of the bio-effective molecule from the fusion molecule.

The fusion molecules of the present invention also can comprise aninsertion sequence and acceptor sequence, wherein either the insertionsequence or the acceptor sequence associates with a bio-effectivemolecule and disassociates from the bio-effective molecule when therespective other sequence of the fusion binds to a cellular marker of apathological condition. In this aspect, the fusion molecule can be usedto target bio-effective molecules, such as drugs, to cells havingspecific pathologies (e.g., cancer cells).

In still another aspect, the fusion molecule of the present invention iscapable of switching from a non-toxic state to a toxic state. Either theinsertion sequence or acceptor sequence may bind to a cellular marker ofa pathology (e.g., such as a tumor antigen). Binding of the marker tothe fusion protein switches the fusion protein from a toxic to anon-toxic state.

In a further aspect, the fusion molecule comprises a molecular switchfor controlling a cellular pathway. The fusion molecule comprises aninsertion sequence and an acceptor sequence and the states of theinsertion sequence and acceptor sequence are coupled, such that thestate of either the inserted sequence or the acceptor sequence modulatesthe activity or expression of a molecular pathway molecule in a cell.The invention can be used to modulate cellular responses using exogenousor endogenous binding molecules (e.g., ligands, small molecules, ions,metabolites, and the like) to transduce a desired signal.

In another aspect, the invention provides a fusion protein comprising aninsertion sequence and an acceptor sequence, wherein either theinsertion sequence or the acceptor sequence binds to a DMA molecule, andwherein DNA binding activity is coupled to the response of therespective other sequence of the fusion molecule to a signal.Preferably, the DNA to which the fusion molecule binds is a nucleic acidregulatory sequence for regulating the activity of another nucleic acidmolecule (e.g., modulating transcription, translation, replication,recombination, supercoiling, etc., of the other nucleic acid molecule).

The invention also provides a sensor molecule comprising an insertionsequence and an acceptor sequence, wherein either the insertion sequenceor acceptor sequence binds to a target molecule and wherein therespective other sequence generates a signal in response to binding.Preferably, the acceptor sequence comprises a deletion and/orduplication at the insertion site.

The invention also provides a combinatorial method for generating any ofthe molecular switches described above. Such an approach provides ameans to systematically examine all or a substantial fraction of allpossible fusions between insertion sequences and acceptor sequences,including ones in which deletions and tandem duplications occur at theinsertion site. Preferably, gives an acceptor sequence comprising agiven number of monomers (e.g., bases, amino acids, etc.), at leastabout the same number of different fusions are generated, and morepreferably, at least about twice this number of fusions are generated.

In one aspect, the method comprises domain insertion, i.e., randomlyinserting an insertion sequence into art acceptor sequence and selectingfor a fusion molecule in which the state of the insertion sequence iscoupled to the state of the acceptor molecule. In another aspect,however, the method comprises generating first and second molecules withdimerization domains and selecting for molecules which dimerize inresponse to a condition, e.g., such as upon binding to a signalingmolecule.

The invention also provides a method for assembling a modulatable fusionmolecule, comprising: randomly inserting an insertion sequence into anacceptor sequence, wherein the insertion sequence and the acceptorsequence each comprise a state (e.g., such as an activity), therebygenerating a fusion molecule, and selecting a fusion molecule whereininsertion couples a change in state of the insertion sequence to achange in the state of the acceptor sequence. In one aspect, an activityof the insertion sequence is modulated, preferably, in response to achange in a state of the acceptor sequence. In another aspect, theactivity of the acceptor sequence is modulated, preferably in responseto a change in the state of the insertion sequence. Insertion of theinsertion sequence into the acceptor sequence, in some cases, maygenerate a new state (e.g., a new activity). The process of randomlyinserting may generate a duplication or deletion at the insertion site,thereby increasing the numbers of types effusions that can be examined.

The invention also provides a method for assembling a multistable fusionmolecule which can switch between at least an active state and a lessactive state, an in some cases, an inactive state. The method comprisesrandomly inserting an insertion sequence into an acceptor sequence,thereby generating a fusion molecule, wherein either the insertionsequence or the acceptor sequence comprises an activity; and wherein therespective other sequence is responsive to a signal. A fusion moleculeis selected in which activity is coupled to the signal such that thefusion molecule switches state in response to the signal. The signal cancomprise binding of a ligand, a change in conformation, a chemical,optical, electrical, magnetic signal, the absence of such conditions,and the like. In one aspect, the method, comprises randomly inserting aninsertion sequence responsive to a signal into an acceptor sequencecomprising an activity, thereby generating a fusion molecule, andselecting for a fusion molecule wherein the activity of foe acceptorsequence is responsive to the signal.

Preferably, the insertion sequence and acceptor sequence comprisepolypeptides and in one aspect, the step of randomly inserting foeinsertion molecule into the acceptor molecule comprises obtaining afirst nucleic acid fragment encoding the insertion polypeptide and asecond nucleic acid fragment encoding the acceptor polypeptide andrandomly inserting the first-nucleic acid fragment into the secondnucleic acid fragment. The method may further comprise the step ofdigesting the second nucleic acid with a nuclease such as DNase I, S1nuclease, mung bean nuclease, a restriction endonuclease, or acombination thereof, shearing the second nucleic acid (e.g.,mechanically), or otherwise treating foe second nucleic acid tointroduce breaks (e.g., exposing the nucleic acid to chemical agentsand/or radiation). The nucleic acid sequence encoding the insertionsequence may also be digested, sheared, or otherwise treated, togenerate random fragments of the insertion sequence. Preferably, suchfragments are inserted at random into the sites of breaks in the nucleicacid sequence encoding the acceptor molecule caused by the nucleasedigestion.

The step of insertion can be repeated a plurality of times with aplurality of first and second nucleic acid molecules, eithersequentially or simultaneously, to generate a library of acceptorpolypeptides comprising randomly inserted insertion polypeptidesequences. The library can be used to identify fusion polypeptideswherein the states of the insertion polypeptide and acceptor polypeptideare coupled, and preferably, responsive to a signal.

In one aspect, the library comprises members comprising insertions withdeletions at the insertion site, insertions with tandem duplications atthe insertion site, and insertions with neither duplications nordeletions.

The invention also provides expression vectors for expression, of thefusion molecules as well as host cells for expressing the fusionmolecules. Host cells can include microorganisms, animal cells, andplant cells. In one aspect, fusion molecules are expressed in one ormore cells of a transgenic organism. Fusion molecules according to theinvention can thus be used to provide a conditional knockout or knock-inof a biomolecule in a cell.

The invention, further provides a method for modulating a cellularactivity comprising providing any of the fusion molecules describedabove, wherein a change in state of at least the insertion sequence orthe acceptor sequence modulates a cellular activity, and wherein thechange in state which modulates the cellular activity is coupled to achange in state of the respective other portion of the fusion molecule.The cellular activity is modulated by changing the state of therespective other portion of the fusion molecule.

In another aspect, the invention provides a method for delivering abio-effective molecule to a cell. The method comprises providing afusion molecule associated with a bio-effective molecule to the cell,the fusion molecule comprising an insertion sequence and an acceptorsequence. Preferably, either the insertion sequence or the acceptorsequence binds to a cellular marker of a pathological condition and uponbinding to the marker, the fusion molecule dissociates from thebio-effective molecule, thereby delivering the molecule to the cell.

In still another aspect, the invention provides a method for deliveringa bio-effective molecule intracellularly. The method comprises providinga fusion molecule associated wife a bio-effective molecule to the cell,the fusion molecule comprising an insertion sequence and an acceptorsequence. Either the insertion sequence or acceptor sequence comprises atransport sequence for transporting the fusion molecule intracellularly.Preferably, release of the bio-effective molecule from the fusionmolecule is coupled to transport of the fusion molecule intracellularly.Preferably, either the inserted sequence or the acceptor sequence iscapable of binding to a biomolecule and binding of the fusion moleculewith the biomolecule transports the fusion molecule intracellularly anddisassociates the bio-effective molecule from the fusion molecule.

The invention also provides a method for modulating a molecular pathwayin a cell. The method comprises providing a fusion molecule to the cell,the fusion molecule comprising an insertion sequence and an acceptorsequence. The states of the insertion sequence and acceptor sequence arecoupled and responsive to a signal, and the state of either theinsertion sequence or the acceptor sequence modulates the activity orexpression of a molecular pathway molecule in the cell. Upon exposure ofthe fusion molecule to the signal, the fusion molecule is thus able tomodulate the molecular pathway.

The invention additionally provides a method for controlling theactivity of a nucleic acid regulatory sequence. The method comprisesproviding a fusion molecule which comprises an insertion sequence andart acceptor sequence, wherein either the insertion sequence or theacceptor sequence responds to a signal, and wherein fee respective othersequence of the fusion molecule binds to the nucleic acid regulatorysequence when fire signal is responded to. Exposing the fusion moleculeto the signal modulates the activity of the nucleic acid regulatorysequence. Types of activities regulated include, but are not limited to,modulating transcription, translation, replication, recombination, orsupercoiling.

The invention also provides a method for generating a conditionalheterodimer, comprising: providing a plurality of randomly bisectedmolecules; each bisected molecule comprising a first portion and asecond portion, wherein the first and second portions are fused to firstand second dimerization domains respectively, and wherein a function ofthe bisected molecule is altered by bisection. By selecting torrestoration of function of a bisected molecule in response to a signal,a conditional heterodimer may be obtained.

In one aspect, a conditional heterodimer is used to conditionallyprovide an activity to a cell. Preferably, the dimerization is mediatedby a signal, such as binding of drug to the dimerization domain suchthat the activity can be triggered by administering a drug to the cell.

BRIEF DESCRIPTION OF THE FIGURES

The objects and features of the invention can be better understood wifeinference to the following detailed description and accompanyingdrawings.

FIGS. 1A-C are schematic diagrams illustrating strategies for generatingmolecular switches according to the invention. FIG. 1A shows a domaininsertion strategy according to one aspect of the invention. FIG. 1Bshows conditional heterodimers according to another aspect of theinvention. FIG. 1C shows a strategy for generating an enzyme:bindingprotein hybrid according to one aspect of the invention. As shown inFIG. 1C, catalytic activity of an enzyme domain of fee fusion moleculeis coupled to binding of the fusion molecule to a signaling protein(protein B).

FIGS. 2A-D show cloning steps in generating libraries of fusion,molecules according to one aspect of the invention. FIG. 2A showspreparation of a nucleic acid encoding an insertion sequence (e.g.,β-lactamase) for subsequent cloning steps. FIG. 2B shows randominsertion of the insertion sequence into acceptor sequences digestedwith, a nuclease. FIG. 2C shows a variation of the insertion methodshown in 2B which comprises Incremental truncation. FIG. 2D is a flowchart illustrating selection of active fusions according to one aspectof the invention.

FIGS. 3A-G illustrate methods of using molecular switches according toaspects of the invention. FIG. 3A shows regulation of gene transcriptionusing a fusion molecule according to one aspect of the invention. FIG.3B shows modulation of a cell signaling pathway according to anotheraspect of the invention. FIG. 3C shows drug delivery mediated by afusion molecule to a cell expressing a marker of a pathology. FIG. 3Dshows the use of fusion molecules for drug transport to an intracellularcompartment. FIG. 3E shows delivery of a conditionally toxic fusionmolecule to a cell. FIG. 3F shows the use of a fusion molecule formetabolic engineering. FIG. 3G shows a fusion molecule according to oneaspect of the invention which functions as a biosensor.

FIG. 4 shows a fusion molecule according to one aspect of the inventionwhich comprises the transferrin domain transport sequence and amethotrexate binding sequence (e.g., such as Dihydrofolate reductase).Outside the cell, the transferrin domain of the ‘Trojan horse’ fusionprotein binds iron and the drug binding domain hinds methotrexate. Thefusion protein interacts with the transferrin receptor and isendocytosed. A decrease in pH in the endosome causes a conformationalchange in the transferrin domain resulting in a conformational change inthe drug binding domains which occurs concomitant with drug release. Thefusion is recycled back outside of the cell to repeat the cycle again.

FIGS. 5A-C show a strategy for engineering a switch molecule bygenerating a conditional heterodimer. FIG. 5A shows bisecting apolypeptide whose function is to be controlled into two fragments thatcannot functionally associate by themselves. FIG. 5B shows selection ofmolecules which functionally associate when fused to dimerizationdomains. FIG. 5C shows dimerization which occurs in response to a signalaccording to one aspect of the invention.

FIGS. 6A-B show strategies to generate libraries of fusion moleculescomprising bisected polypeptides fused to oligomerization domains. FIG.6A shows a method for generating libraries of such molecules. FIG. 6Bshows the addition of dimerization domains.

FIG. 7A shows the frequency of active heterodimers of Neo identifiedfrom a library of fusion molecules whose assembly is assisted byantiparallel leucine zippers. FIG. 7B is a graph summarizing sequencedata obtained from libraries comprising heterodimers as in FIG. 7A.Sequences falling on the diagonal line in the graph have no overlap ordeletion between fragments. Sequences of heterodimers above the linehave overlapping sequences, while those below the line have deletedamino acids. In a library without a flexible linker, sequencing ofsixteen randomly selected colonies from kanamycin plates resulted in theidentification of ten different heterodimers of Neo (indicated by thelarge cross) whose assembly is assisted by antiparallel leucine sappers,in a library with a GSGG flexible linker, sequencing of six randomlyselected colonies from kanamycin plates resulted in the identificationof four different heterodimers of Neo (indicated by the thin-linecross).

FIG. 8 shows the effect of sugars on a T164-165 β-lactamase: maltosebinding protein (MBP) fusion's hydrolysis of nitrocefin. The fusioncomprises an insertion of β-lactamase amino acid sequences into an MBPacceptor polypeptide with a tandem duplication of amino acids 164-165 ofMBP at the insertion site. The velocity of nitrocefin hydrolysis with150 μM nitrocefin and 5 mM of the indicated sugars was compared to thevelocity without any sugar. Sugars know not to bind wildtype MBP(sucrose) and those that bind to MBP, but do not introduce aconfrontational change (maltitol and β-cyclodextrin) did not have asignificant effect on nitrocefin hydrolysis. All sugars known to bind towildtype MBP and induce a conformational change (maltose, maltotrioseand maltohexose) increase the rate of hydrolysis by approximately 40%.

DETAILED DESCRIPTION

The invention provides molecular switches which couple external signalsto functionality and to methods of making and using the same. Theswitches according to the invention can be used, for example, toregulate gene transcription, target drug delivery to specific cells,transport drugs intracellulary, control drag release, provideconditionally active proteins, perform metabolic engineering, andmodulate cell signaling pathways. Libraries comprising the switches andexpression vectors and host cells for expressing the switches are alsoprovided.

DEFINITIONS

The following definitions are provided for specific terms which are usedin the following written description.

As used herein, a “molecular switch” refers to a molecule whichgenerates a measurable change in state in response to a signal. In oneaspect, a molecular switch is capable of switching from at least onestate to at least one other state in response to the signal. Preferably,when a portion of the molecule responds to the signal, the portionbecome activated (i.e., turns “ON”) or inactivated (i.e., turns “OFF”).In response to this change in state, the state of another portion of thefusion molecule will change (e.g., turn ON or OFF), ha one aspect, aswitch molecule tarns ON one portion of the molecule when anotherportion is turned OFF. In another aspect, the switch turns ON oneportion of the molecule, when the other portion is turned ON. In stillanother aspect, the switch molecule tarns OFF one portion of themolecule when the other portion is turned ON. In a farther aspect, theswitch molecule turns OFF, when the other portion is turned OFF. In someaspects of the invention, a molecular switch exists in more than twostates, i.e., not simply ON or OFF. For example, a portion of the fusionmolecule may display a series of states (e.g., responding to differentlevels of signal), while another portion of the fusion molecule respondsat each state, with a change in one or more states. A molecular switchalso can comprise a plurality of fusion molecules responsive to a signaland which mediate a function by changing the state of at least a portionof the molecule (preferably, in response to a change in state of anotherportion of the molecule). While the states of individual fusionmolecules in the population may be ON or OFF, the aggregate populationof molecules may not be able to mediate the function unless a thresholdnumber of molecules switch states. Thus, the “state” of the populationof molecules may be somewhere in between ON or OFF depending on thenumber of molecules which have switched states. In one aspect, amolecular switch comprises a heterogeneous population of fusionmolecules comprising members which switch states upon exposure todifferent levels of signal. In other aspects of the invention, however,the state of a single molecule may be somewhere in between ON or OFF.For example, a molecule may comprise a given level of activity, abilityto bind, etc., in one state which is switched to another given level ofactivity, ability to bind, etc., in another state (i.e., an activity,ability to bind, etc., measurably higher or lower than the activity,ability to bind, etc, observed in previous state).

As used herein, a “state” refers to a condition of being. For example, a“state of a molecule” or a “state of a portion of a molecule” can be aconfirmation, binding affinity, or activity (e.g., including, but notlimited to, ability to catalyze a substrate; ability to emit light,transfer electrons, transport or localize a molecule, modulatingtranscription, translation, replication, supercoiling, and the like).

As defined herein, a molecule, or portion thereof, whose state is“activated” refers to a molecule or portion thereof which performs anactivity, such as catalyzing a substrate, emitting light, transferringelectrons, catalyzing a substrate, transporting or localizing amolecule; changes conformation; binds to a molecule, etc.

As defined herein, a molecule, or portion thereof, whose state is“inactivated” refers to a molecule or portion thereof which is, at leasttemporarily, unable to perform an activity or exist in a particularstate (e.g., bind to a molecule, change conformation).

As used herein, “coupled” refers to a state which is dependent onanother state such that a measurable change in the other state isobserved. As used herein, “measurable” refers to a that is significantlydifferent from a baseline or a previously existing state as determinedin a suitable assay using routine statistical methods (e.g., settingp<0.05).

As used herein, “a signal” refers to a molecule or condition feat causesa reaction. Signals include, but are not limited to, the presence,absence, or level, of molecules (nucleic acids, proteins, peptides,organic molecules, small molecules), ligands, metabolites, ions,organelles, cell membranes, cells, organisms (e.g., pathogens), and thelike; as well as the presence, absence, or level of chemical, optical,magnetic, or electrical conditions, and can include conditions such asdegrees of temperature and/or pressure. A chemical condition can includea level of ions, e.g., pH.

As used herein, “responsive to a signal” refers to a molecule whosestate is coupled to the presence, absence, or level of the signal.

As used herein, “an insertion sequence” refers to a polymeric sequencewhich is contained within another polymeric sequence (e.g., an “acceptersequence”) and which conditionally alters the state of the otherpolymeric sequence. An insertion sequence or acceptor sequence cancomprise a polypeptide sequence, nucleic acid sequence (DNA sequence,aptamer sequence, RNA sequence, ribozyme sequence, hybrid sequence,modified or analogous nucleic acid sequence, etc), carbohydratesequence, and the like.

As used herein, “multistable” refers to a fusion molecule which iscapable of existing in at least two states.

As used herein, “bistable” refers to a fusion molecule capable ofexisting in two states.

As used herein, “range of states” refers to a series of states in whicha fusion molecule can exist. For example, a range of states can comprisea range of binding activities, a range of light-emitting activities, arange of catalysis efficiencies, and the like.

As used herein, “a change in state” refers to a measurable difference ina state of being of a molecule, as determined by an assay appropriatefor that state.

As used herein, “a graded response” refers to the ability of a fissionmolecule to switch to a series of states in response a particularthreshold signal.

As used herein, “modulates” or “modulated” refers to a measurable changein a state or activity or function. Preferably, where an activity isbeing described, “modulated” refers to an, at least 2-fold, at least5-fold, at least 10-fold, at least 20-fold or higher, increase ordecrease in activity, or an at least 10%, at least 20%, at least 30%, atleast 40% or at least 50% increase or decrease in activity. However,more generally, any difference which is measurable and statisticallydifferent from a baseline is encompassed within the term “modulated”.

As used herein, a “less active state” is a state which is at least about2-fold less active compared to a given reference state as measured usingan assay suitable for measuring that state, or about at least 10%, atleast about 20%, at least about 30%, at least about 40%, at least about50%, at least about 60%, at least about 70%, at least about 80%, atleast about 90% or at least about 100% less active. More generally, artydecrease which is measurable and statistically different from baselineis encompassed within the term “less active state”.

As used herein, a “less toxic state” refers to & measurable increase inthe LD₅₀ (i.e., lethal dose which has a 50% probability of causingdeath) or LC₅₀ (i.e., lethal concentration which has a 50% probabilityof causing death). Preferably, a less toxic state is one which isassociated with an at least about 10% increase, at least about 20%, atleast about 30%, at least about 40%, at least about 50%, at least about60%, at least about 70%, at least about 80%, at least about 90% or atleast about 100% increase in LD₅₀ or LC₅₀.

As used herein, “a bio-effective molecule” refers to bioactive moleculewhich can have an affect on the physiology of a cell or which can beused to image a cell. In one aspect, a “bio-effective molecule” is apharmaceutical agent or drug or other material that has a therapeuticeffect on the cell.

As used herein, “a cellular marker of a pathological condition” refersto a molecule which is associated with a cell, e.g., intracellularly orextracellularly, and whose presence or level correlates with thepresence of the disease, i.e., the marker is found in, or on cells, oris secreted by cells, exhibiting the pathology at levels which aresignificantly different than observed for cells not exhibiting thepathology.

As used herein, “a molecular pathway molecule” refers to a moleculewhose activity and/or expression affects the activity and/or expressionof at least two other molecules. Preferably, a molecular pathwaymolecule is a molecule involved in a metabolic or signal transductionpathway. A pathway molecule can comprise a protein, polypeptide,peptide, small molecule, ion, cofactor, organic and inorganic molecule,and the like.

As used herein, “modulating a molecular pathway” refers to a change inthe expression and/or activity of at least one pathway molecule.

As used herein, “at an insertion site” of a nucleic acid molecule refersto from about 1 to 21 nucleotides immediately flanking the insertionsite.

As used herein, “randomly inserting” refers to insertion at non-selectedsites in a polymeric sequence. In one aspect, “random insertion” refersto insertion that occurs in a substantially non-biased fashion, i.e.,there is a substantially equal probability of inserting between membersof any pairs of monomers (e.g., nucleotides or amino acids) in anacceptor molecule comprising a given number of monomeric sequences.However, in another aspect, random insertion has some degree of bias,e.g., there is a greater than equal probability of inserting atdifferent sites. Minimally, the probability of insertion at a site in anacceptor sequence is greater than zero but less than one.

As used herein, “a new activity” refers to an activity which is notfound in either donor or acceptor sequences. Generally, fusion moleculesaccording to the invention comprise a new activity in that the activityof the acceptor sequence or insertion sequence is newly coupled to thestate of the respective other portion of the sequence. An insertion oracceptor sequence also may comprise a catalytic site which responds to(e.g., catalyzes) a substrate provided in the form of the respectiveother portion of the fusion molecule, thereby producing a fusionmolecule which comprises an activity present in neither the originalcatalytic site or the substrate (e.g., such as the ability toself-cleave in the presence of a signal).

As used herein, “a nuclear regulatory sequence refers to” a nucleic acidsequence which is capable of modulating the activity of another nucleicacid in cis or in trans. Types of activities regulated include, but arenot limited to, modulating transcription, translation, replication,recombination, or supercoiling. A nucleic acid regulatory sequence caninclude promoter elements, operator elements, repressor elements,enhancer sequences, ribosome binding sites, IRES sequences, origins ofreplication, recombination hotspots, topoisomerase binding sequences,and the like.

As used herein, “altered by bisection” refers to a change in state uponfragmenting a polypeptide into two pieces. The term “bisection” does notimply that the polypeptide is divided into fragments of equal size;rather fragments can be generated by cleaving anywhere along the lengthof the primary sequence of the amino acid.

As used herein, “selecting for restoration of function or state” refersto selection for restoration of a function or state which issufficiently similar to that of the original function under assayconditions suitable for evaluating the function or state. As usedherein, “sufficiently similar” refers to a state that can achieve theoriginal function in an effective manner. For example, when thefunction/state is binding, restoration of function/state can beevaluated by generating Scatchard plots and/or determining K_(d). Whenthe function/state is the ability of a molecule to generate light,restoration can be measured spectrophotometrically, for example.

As used herein, a “modification” of a polypeptide refers to an addition,substitution or deletion of one or more amino acids in a polypeptidewhich does not substantially alter the state of the polypeptide. Forexample, where a state is art activity of a polypeptide, a modificationresults in no more than a 10% decrease or increase in the activity ofthe polypeptide, and preferably no more than a 5% decrease or increasein the activity of the polypeptide.

Fusion Molecules Domain Insertion

In one aspect, a fusion molecule is provided which comprises aninsertion sequence and an acceptor sequence which contains the insertionsequence (see, FIG. 1B). Preferably, the insertion sequence andacceptor-sequence are polymeric molecules, e.g., such as polypeptides ornucleic acids. More preferably, both the insertion sequence and acceptorsequence are capable of existing in at least two states and the state ofthe insertion sequence is coupled to the state of the acceptor sequenceupon fusion, such that a change in state in either the insertionsequence or acceptor sequence will result in a change in state ofrespective other portion of the fusion. A “state” can be a conformation;binding affinity; ability or latent ability to catalyze a substrate;ability or latent ability to emit light; ability or latent ability totransfer elections; ability or latent ability to withstand degradation(e.g., by a protease or nuclease); to modulate transciption; ability orlatent ability to modulate translation; ability or latent ability tomodulate replication; ability or latent ability to initiate or mediaterecombination or supercoiling; or otherwise perform a function; and thelike.

Preferably, the change in state is triggered by a signal to which thefusion molecule is exposed, e.g., such as the presence, absence, oramount of a small molecule, ligand, metabolite, ion, organelle, cellmembrane, cell, organism (e.g., such as a pathogen), temperature change,pressure change, and the like, to which the fusion molecule binds; achange in a condition, such as pH, or a change in the chemical, optical,electrical, or magnetic environment of the fusion molecule. In oneaspect, a fusion molecule functions as an ON/OFF switch in response to asignal (e.g., changing from one state to another). For example, when aninsertion sequence or acceptor sequence of the fusion molecule binds toa ligand, the respective other half of the fusion may change state(e.g., change conformation, bind to a molecule, release a molecule towhich it is bound, catalyze a substrate or stop catalyzing a substrate,emit light or stop emitting light, transfer electrons or stoptransferring electrons, activate or inhibit transcription, translation,replication, etc).

However, fusion molecules according to the invention also can be used togenerate graded responses. In this scenario, a fusion molecule canswitch from a series of states (e.g., more than two different types ofconformations, levels of activity, degrees of binding, levels of lighttransmission, electron transfer, transcription, translation,replication, etc). Preferably, the difference in state is one which canbe distinguished readily from other states (e.g., there is a significantmeasurable difference between one state and any other state, asdetermined rising assays appropriate for measuring that state).

More generally, a molecular switch is one which generates a measurablechange in state in response to a signal. For example, a molecular switchcan comprise a plurality of fusion molecules each responsive to a signaland for mediating a function in response to a change in state of atleast a portion of the molecule. As above, preferably, this change ofstate occurs in response to a change in state of another portion of themolecule. While the states of individual fusion molecules in thepopulation may be ON or OFF, the aggregate population of molecules maynot be able to mediate the function, unless a threshold number ofmolecules switch states. Thus, the “state” of the population ofmolecules may be somewhere in between ON or OFF, depending on the numberof molecules which have switched states. This provides an ability tomore precisely tune a molecular response to a signal by selecting formolecules which respond to a range of signals and modifying thepopulation of fusion molecules to provide selected numbers of fusionmolecules, providing an aggregate switch which respond to a narrow rangeor wider range of signal as desired. Thus, in one aspect, aheterogeneous population of fusion molecules is provided comprisingmembers which respond to different levels or ranges of signals.Individual fusion molecules also may exist in states intermediatebetween ON or OFF; e.g., having a given level of activity, ability tobind to a molecule in one state and a measurably higher or lower levelof activity, ability to bind, etc., in a different state.

Insertion Sequences

The size of the insertion will vary depending on the size of aninsertion sequence required to confer a particular state on theinsertion sequence without significantly disrupting the ability of theacceptor molecule into which it is inserted to change state. Preferably,the affect of the insertion is to couple the change in state of theacceptor molecule to a change in state of the insertion molecule or visaversa.

Generally, for polypeptide insertions, the size of the insertionsequence can range from about two amino acids to at least about 120amino acids. In one aspect, the insertion comprises a domain sequencewith a known characterized activity (e.g., a portion of a protein inwhich bioactivity resides); however, in other aspects, the insertionsequence comprises an entire protein sequence.

In one aspect, the insertion sequence is a polypeptide whose foldedconformation is such feat the N- and C-termini are “on the same face” ofa fusion molecule comprising the insertion sequence.

Acceptor Sequences

Generally, there are no constraints on the size or type of acceptorsequence which can be used. However, in one aspect, an acceptor sequenceis a polypeptide whose state resides in a discontinuous domain of aprotein (e.g., the amino acids involved in conferring the state/activityof the acceptor sequence are sot necessarily contiguous in the primarypolypeptide sequence) (see, e.g., as described in Russell and Ponting,1998, Curr. Opin. Struct. Biol. 8:364-371, and Jones, et al., 1998,Protein Sci 7; 233-42).

Suitable polypeptides for acceptor molecules can be identified usingdomain assignment algorithms such as are known in the art (e.g., such asthe PUU, DETECTIVE, DOMAK, and DomainParser, programs). For example, aconsensus approach may be used as described in Jones, et al., 1998,supra. Information also can be obtained from a number of molecularmodeling databases such as the NTH Molecular Modelling Homepage,accessible at http://cmm.info.nih.gov/modeling/pdb_at_a_glance.html; orthe 3Dee Database described by Dengler, et al., 2001, Proteins 42(3):332-44. However, the most important criteria used for selecting asequence is its function, e.g., the desired state parameters of thefusion molecule.

However, in a former aspect, no pre-screening is done and an acceptorsequence is selected simply on the basis of a desired activity. Thepower of the methods according to the invention is that they rely oncombinatorial screening to identify any, and preferably, all,combinations of insertions that produce a desired coupling in states ofacceptor and donor molecules.

Domain Sequences

In one aspect, the insertion sequence or acceptor sequence comprises a“domain” sequence having a known state. Domains can be minimalsequences, such as are known in the art, which are associated with aparticular-known state or can be an entire protein comprising the domainor a functional fragment thereof.

Minimal domain sequences can be defined by site-directed mutagenesis ofa sequence having a desired state to determine the minimum amino acidsnecessary to confer the existence of the state under the appropriateconditions (e.g., such as a minimal binding site sequence or a minimumsequence necessary for catalysis, light emission, etc.). As discussedabove, minimal domain sequences also can be defined virtually, usingalgorithms to identify consensus sequences- or areas of likely proteinfolding. Once a domain sequence has been identified, it can be modifiedto include additional sequences, as well as insertions, deletions, andsubstitutions of amino acids so long as they do not substantially affectthe state of the domain sequence. While domain sequences can be obtainedusing nucleic acids encoding appropriate fragments of polypeptides, theyalso can be synthesized, for example, based on a predicted consensussequence for a class of molecules which is associated with a particularstate. However, as discussed above, in some cases it may be desirable toprovide the domain sequence in the form of a native protein comprisingthe domain.

Suitable domain sequences include extracellular domains which areportions of proteins normally found outside of the plasma membrane of acell. Preferably, such domains bind to bio-effective molecules. Forexample, an extracellular domain can include the extracytoplasmicportion of a transmembrane protein, a secreted protein, a cell surfacetargeting protein, a cell adhesion molecule, and the like. In oneaspect, an extracellular domain is a clustering domain, which, uponactivation by a bio-effective molecule will dimerize or oligomerize withother molecules comprising extracellular domains.

Intracellular domains also can serve as insertion sequences or acceptorsequences. As used herein, “an intracellular domain” refers to a portionof a protein which generally resides inside of a cell with respect tothe cellular membrane. In one aspect, an intracellular domain is onewhich transduces an extracellular signal into an intracellular response.For example, an intracellular domain can comprise a proliferation domainwhich signals a cell to enter mitosis (e.g., such as domains from Jakkinase polypeptides, Il-2 receptor β and/or gamma chains, and the like).Other transducer sequences include sequences from the zeta chain of theT cell receptor or any of its homologs (e.g., the eta chain, Fc epsilonR1-gamma and -62 chains, MB1 chain, B29 chain, and the like), CD3polypeptides (gamma, beta and epsilon), syk family tyrosine kinases(Syk, ZAP 70, and the like), and src family tyrosine kinases (Lck, Fyn,Lyn, and the like).

A transmembrane domain also can be used as an insertion sequence oracceptor sequence. Preferably, a transmembrane domain is able to crossthe plasma membrane and can, optionally, transduce an extracellularsignal into an intracellular response. Preferred transmembrane sequencesinclude, but are not limited to, sequences derived from CD8, ICAM-2,IL-8R, CD4, LFA-1, and the like.

Transmembrane sequences also can include GPI anchors, e.g., such as theDAP sequence (PNKGSGTTSGTTRLLSGHTCFTLTGLLGTLVTMGLLT) (see, e.g., Homans,et al, 1988, Nature 333(6170): 269-72; Moran, et al, 1991, J. Biol.Chem. 266: 1250); myristylation sequences (e.g., such as the srcsequence MGSSKSKPKDPSQR) (see Cross, et al., 1984, Mol. Cell. Biol.4(9): 1834; Spencer, et al., 1993, Science 262: 1019-1024); andpalmitoylation sequences (e.g., such as the GRK6 sequenceLLQRLFSRQDCCGNCSDSEEELPTR).

Either the insertion sequence or the acceptor sequence can be alocalization sequence for localizing a molecule comprising the sequenceintracellularly. In one aspect, the localization sequence is a nuclearlocalization sequence. Generally, a nuclear localization sequence is ashort, basic sequence that serves to direct a polypeptide in which itoccurs to a cell's nucleus (Laskey, 1986, Ann. Rev. Cell. Biol.2:367-390; Bonnerot, et al., 1987, Proc. Natl. Acad. Sci. USA 84;6795-6799; Galileo, et al., 1990, Proc. Natl. Acad. Sci. USA 87:458-462, 1990). Suitable nuclear localization sequences include, but arenot limited to, the SV40 (monkey virus) large T Antigen sequence(PKKKKKV) (see, e.g., Kalderon, 1984, et al., Cell 39: 499-509); thehuman retinoic acid receptor nuclear localization signal (ARRRRP); NF κβp50 sequence (EEVQRKRQKL) (Ghosh et al., 1990, Cell 62: 1019); the NF κBp65 sequence (EEKRKRTYE) (Nolan et al., 1991, Cell 64:961); andnucleoplasmin (Ala Val Lys Arg PAATLKKAGQAKKKKLD) (Dingwall, et al.,1982, Cell 30:449-458).

The localization sequence can comprise a signaling sequence forinserting at least a portion of the fusion molecule into the cellmembrane. Suitable signal sequences include residues 1-26 of the IL-2receptor beta-chain (see, Hatakeyama et al., 1989, Science 244: 551; vonHeijne et al, 1988, Eur. J. Biochem. 174: 671); residues 1-27 of theinsulin receptor β chain (see, Hatakeyama, et al., 1989, supra);residues 1-32 of CD8 (Nakauchi, et al., 1985, PNAS USA 82: 5126) andresidues 1-21 of ICAM-2 (Staunton, et al., 1989, Nature (London) 339:61).

The localization sequence also can comprise a lysosomal targetingsequence, including, for example, a lysosomal degradation sequence suchas Lamp-2 (KFERQ) (see, e.g., Dice, 1992, Am. N.Y. Acad. Sci. 674: 58);a lysosomal membrane sequence from Lamp-1(MLIPIAGFFALAGLVLIVLIAYLIGRKRSHAGYOTI) (e.g., Uthayakumar, et. al.,1995, Cell. Mol. Biol. Res. 41: 405) or Lamp-2(LVPIAVGAALAGVLILVLLAYFIGLKHHHAGYEQF) (e.g., Konecki et al., 1994,Biochem. Biophys. Res. Comm. 205: 1-5).

Alternatively, the localization sequence can comprise a mitochondriallocalization sequence, including, hut not limited to: mitochondrialmatrix sequences, such as the MLRTSSLFTRRVQPSLFSRNILRLQST of yeastalcohol dehydrogenase III (Schatz, 1987, Eur. J. Biochem. 165: 1-6);mitochondrial inner membrane sequences, such as theMLSLRQSIRFFKPATRTLCSSRYLL sequence of yeast cytochrome c oxidase subunitIV (Schatz, 1987, supra); mitochondrial intermembrane space sequences,such as the MFSMLSKRWAQRTLSKSFYSTATGAASKSGKLTQKLVTAGVAAAGITASTLLYADSLTAEAMTA sequence of yeast cytochrome c1 (Schatz, 1987, supra); ormitochondrial outer membrane sequences, such as theMKSFITRNKTAILATVAATGTAIGAYYYYNQLQQQQQRGKK sequence of yeast 70 kD outermembrane protein (see, e.g., Schatz, supra).

Other suitable localization sequences include endoplasmic reticulumlocalizing sequences, such as KDEL from calreticulin (e.g., Pelham,1992, Royal Society London Transactions B: 1-10) or the adenovirusE3/19K protein sequence LYLSRRSFIDEKKMP (Jackson et al., 1990, EMBO, J.9: 3153); and peroxisome targeting sequences, such as the peroxisomematrix sequence (SKL) from Luciferase (Keller et al., 1987, Proc. Natl.Acad. Sci. USA 4:3264).

In another aspect, the insertion sequence or acceptor sequence comprisesa secretory signal sequence capable of effecting the secretion of thefusion molecule from a cell (see, e.g., Silhavy, et al, 1985, MicrobiolRev. 49: 398-418). This may be useful for generating a switch moleculewhich can affect the activity of a cell other than a host cell in whichit is expressed. Suitable secretory sequences, include, but are notlimited to the MYRMQLLSCIALSLALVTNS sequence of IL-2 (Villinger, et al.,1995, J. Immunol 155: 3946); the MATGSRTSLLLAFGLLCLPWLQEGSAFPT sequenceof growth hormone (Roskam et al, 1979, Nucleic Acids Res. 7:30); theMALWMRLLPLLALLALWGPDPAAAFVN sequence of preproinsulin (Bell, et al.,1980, Nature 284:26); the influenza HA protein sequence,MKAKLLVLLYAFVAGDQI (Sekiwawa, et al., Proc. Natl. Acad. Sci. USA80-3563); or the signal leader sequence from the secreted cytokine IL4,MGLTSQLLPPLFFLLACAGHFVHG.

In a further aspect, the insertion sequence or acceptor sequencecomprises a domain for binding a nucleic acid. The domain can comprise aDNA binding polypeptide or active fragment thereof from a prokaryote oreukaryote. For example, the domain can comprise a polypeptide sequencefrom a prokaryotic DNA binding protein such as gp 32; a domain from aviral protein, such as the papilloma virus E2 protein; or a domain froma eukaryotic protein, such as p53, Jun, Fos, GCN4, or GAL4. Novel DNAbinding proteins also can be generated by mutagenic techniques (see,e.g., as described in U.S. Pat. No. 5,198,346).

The insertion sequence or acceptor sequence also can comprise the Ca²⁺binding domain of a Ca+ binding protein such as calmodulin, paralbumin,troponin, annexin, and myosin or the ligand domain of a binding proteinsuch as avidin, concanavalin A, ferritin, fibronectin, animmunoglobulin, a T Cell Receptor, an MHC Class I or Class II molecule,a lipid binding protein, a metal binding protein, a chaperone, aG-Protein Coupled Receptor, and the like.

In addition, the insertion or acceptor sequence can comprise thetransport domain of a transport protein such as hemerythrin, hemocyanin,hemoglobin, myoglobin, transferrin, lactoferrin, ovotransferrin, maltosebinding protein and transthyretrin.

In another aspect, the insertion or acceptor sequence can comprise theactive domain of a blood coagulation protein (e.g., a domain whichmediates blood clotting). Exemplary blood clotting proteins include, butare not limited to: decorsin, factor IX, factor X, kallikrein,plasmin/plasminogen, protein C, thrombin/prothrombin, and tissue-typeplasminogen activator.

In still another aspect, the insertion or acceptor sequence can comprisethe active domain of an electron transport protein (e.g., a domain whichconfers electron transport activity on a protein). Electron transportproteins include, but are not limited to, amicyanin, azurin, acytochrome protein, ferrodoxin, flavodoxin, glutaredoxin, methylaminedehydrogenase, plastocyanin, rubredoxin, and thioredoxin.

In a former aspect, the insertion sequence or acceptor sequencecomprises me catalytic and/or substrate binding site of an enzyme.Suitable enzymes from which such sites are selected include:β-lactamase; acetylcholinesterase; an amylase; barnase; deaminase; akinase (e.g., such as a tyrosine kinase or serine kinase); aphosphatase; an endonuclease; an exonuclease; an esterase; an enzymeinvolved in a metabolic pathway (e.g., fructose-1,6-bisphosphatase); aglycosidase; a heat shock protein; a lipase; a lysozyme; aneuramidase/sialidase; a phospholipase; a phosphorylase; apyrophosphatase; a ribonuclease; a thiolase; a polymerase; an isomerase(such as a mutase; triosephosphate isomerase, xylose isomerase,topoisomerase, gyrase); a lyase (such as aconitase, carbonic anhydrase,pyruvate decarboxylase); an oxidoreductase (such as alcoholdehydrogenase, aldose reductase, a catalase, cytochrome C peroxidase,cytochrome p450, a dehydrogenase, dihydrofolate reductase,glyceraldehydes-3-phosphate dehydrogenase, a hydroxybenzoatehydroxylase, a lactate dehydrogenase, a peroxidase, and a superoxidedismutase); a protease (such as actinidin, α-lytic protease,aminopeptidase, carboxypeptidase, chymosin, chymotrypsin, elastase,endopeptidase, endothiapepsin, HIV protease, Hannuka factor, papain,pepsin, rennin, substilisin, thermolysin, thermitase, and trypsin), atransferase (such as acetyltransferase, aminotransferase,carbamoyltransferase, dihyrolipoamide acetyltransferase, dihydrolipoyltransacetylase, Dihydrolipoamide Succinyltransferase, a nucleotidyltransferase, DNA methyltransferase, formyltransferase,glycosyltransferase, a phosphotransferase, a phosphoribosyltransferase),a dehalogenase, a racemase, and the like.

The catalytic domain also can be a rhodanese homology domain such asforms the active site in various phosphatases and transferases (e.g.,such as found in the Cdc25 family of protein dual specificityphosphatases, the MKP1/PAC1 family of MAP-kinase phosphatases, thePyp1/Pyp2 family of MAP-kinase phosphatases, and certain ubiquitinhydrolases) (see, e.g., Hofmann, et al., 1998, J. Mol. Biol.282:195-208).

Still other domains can include toxins such as cardiotoxin, conotoxin,erabutoxin, momorcharin, momordin, and ricin.

Other domains include, but are not limited to, signaling domains such asthe FHA domain, found in protein kinases and transcription factors suchas fork head, DUN1, RAD53, SPK1, cds1, MEK1, KAFP, NIPP1, Ki-67, fraH,and KIAA0170 (see, e.g., Hofmann and Bucher, 1995, Trends Biochem. Sci.20: 347-349); the death domain, a heterodimerization domain present inproteins involved in apoptotic signal transduction and the NFkβ pathway(such as TNFR1, FAS/APO1, NGFR, MORT1/FADD, TRADD, RIP, ankyrin, MyD88,unc-5, unc-44, DAP-kinase, Rb-binding p84, pelle, NFkβ, and tubepolypeptides) (see. e.g., Hofmann and Tschopp, 1995, FEBS Lett. 371:321-323); and the G-protein desensitization domain (found in ARK1, GRK,G-protein coupled receptor kinases, eg1-10, GAIP, BL34 SST2, flbA, RGP3,RGP4Human G0/G1 switch regulatory protein 8, Human B-cell activationprotein BL34, and G-protein coupled receptor kinases) (see, e.g.,Hofmann and Bucher, “Conserved Sequence Domains in Cell Cycle RegulatoryProteins”, abstract presented at the joint ISREC/AACR meeting “Cancerand the Cell cycle”, January 1996 in Lausanne).

In one aspect, either the insertion or the acceptor sequence is alight-emitting polypeptide domain such as one obtained from a GreenFluorescent Protein, or modified, or mutant form thereof (collectivelyreferred to as a “GFP”). The wild-type GFP is 238 amino acids in length(Prasher, et al., 1992, Gene 111(2): 229-233; Cody et al., Biochem.32(5):1212-1218 (1993); Ormo, et al, 1996, Science 273: 1392-1395; andYang, et al., 1996, Nat. Biotech. 14: 1246-1251). Modified forms aredescribed in WO 98/06737 and U.S. Pat. No. 5,777,079, GFP deletionmutants also can be made. For example, at the N-terminus, it is knownthat only the first amino acid of the protein may be deleted withoutloss of fluorescence, while at the C-terminus, up to 7 residues can bedeleted without loss of fluorescence (see, e.g., Phillips, et al., 1997,Current Opin. Structural Biol. 7: 821).

The insertion sequence or acceptor sequence additionally can comprisethe light-reactive portion of a photoreceptor such asbacteriochlorophyll-A, bacteriorhodopsin, photoactive yellow protein,phycocyanin, and rhodopsin.

Additional domain sequences include ligand-binding domains ofligand-binding proteins. Such proteins include, bat not limited to:biotin-binding proteins, lipid-binding proteins, periplasmic bindingproteins (e.g. maltose binding protein), lectins, serum albumins,immunoglobulins, T Cell Receptors, inactivated enzymes,pheromone-binding proteins, odorant-binding proteins,immunosuppressant-binding proteins (e.g., immunophilins such ascyclophilins and FK506-binding proteins), phosphate-binding proteins,sulfate-binding proteins, and the like. Additional binding proteins aredescribed in De Wolf and Brett, 2000, Pharmacological Reviews 52(2):207-236.]

The domain sequences of the proteins described above are known in theart and can be obtained from a database such as available at the NTHMolecular Modelling Homepage, accessible athttp://cmm.info.nih.gov/modeling/pdb_at_a_glance.html.

The insertion and acceptor sequences can be selected from any of thedomain sequences described above and can be of like kind (e.g., bothcatalytic, sites, both binding domains, both light emitting domains) orof different kind (e.g., a catalytic site and a binding site, as shownin FIG. 1C; a binding site and a light emitting domain; etc.). Thedomain sequences can be the minimal sequences required to confer a stateor activity or can comprise additional sequences. Other insertion andacceptor sequences can be derived from known domain sequences or fromnewly identified sequences. Such sequences are also encompassed withinthe scope of the instant invention.

Exemplary Fusion Molecules

In one aspect, the insertion sequence or the acceptor sequence localizesthe fusion molecule intracellularly. Preferably, intracellularlocalization is coupled to the binding of the fusion molecule to abio-effective molecule.

In another aspect, the invention provides a fusion protein comprising aninsertion sequence and an acceptor sequence, wherein either the insertedsequence or the acceptor sequence binds to a DNA molecule, and whereinDNA binding activity is coupled to the response of the respective othersequence of the fusion molecule to a signal.

The fusion molecule also can comprise an insertion sequence and acceptorsequence, wherein either the inserted sequence or the acceptor sequenceassociates with a bio-effective molecule, and disassociates from thebio-effective molecule, when the respective other sequence of the fusionbinds to a cellular marker of a paralogical condition. Such markers cancomprise polypeptides, nucleic acids, glycoproteins, lipids,carbohydrates, small molecules, metabolites, pH, ions and the like.Examples of cellular markers of pathological conditions include, but arenot limited to cancer-specific or tumor-specific antigens,pathogen-encoded polypeptides (e.g., viral-, bacterial-, protist-, andparasite-encoded polypeptides) as are known in the art.

In still another aspect, the fusion molecule is capable of switchingfrom a non-toxic state to a toxic state. Either the insertion sequenceor acceptor sequence may bind to a cellular marker of a pathology (e.g.,such as a tumor antigen). Binding of the marker to the fusion proteinswitches the fusion protein from a non-toxic state or a less toxic stateto a toxic state. Similarly, a marker of a healthy cell could be used asa trigger to switch a fusion molecule from a toxic state to a non-toxicstate, or to a less toxic state.

In a further aspect, the fusion molecule comprises a molecular switchfor controlling a cellular pathway. The fusion molecule comprises aninsertion sequence and an acceptor sequence and the states of theinsertion sequence and acceptor sequence are coupled, such that thestate of either the insertion sequence or the acceptor sequencemodulates the activity or expression of a molecular pathway molecule ina cell. Preferably, modulation of activity or expression occurs when therespective other portion of the fusion molecule responds to a signal,e.g., binds to an exogenous or endogenous binding molecule (e.g.,ligands, small molecules, ions, metabolites, and the like), responds toelectrical or chemical properties of a cell, or responds to the opticalenvironment in which a cell is found (e.g., responding to the presenceor absence of particular wavelength(s) of light).

The invention also provides a sensor molecule comprising an insertionsequence and an acceptor sequence, wherein either the insertion sequenceor acceptor sequence binds to a target molecule and wherein therespective other sequence generates a signal in response to binding.Preferably, the acceptor sequence comprises a deletion and/orduplication at the insertion site.

It should be obvious to those of skill in the art that these are onlyexemplary combinations of insertion and acceptor sequences that can beused.

Additional Sequences

Fusion molecules can comprise domain sequences in addition to insertionand acceptor sequences. Such domains can comprise states which may ormay not be coupled with the states of the other portions of the fusionmolecule.

Additional sequences also can be included as part of the fusion moleculewhich do not alter substantially the states of the insertion sequence oracceptor sequence portion of the fusion molecule. For example, affinitytag sequences can be provided to facilitate the purification orisolation of the fusion molecule. Thus, His6 tags can be employed (foruse with nickel-based affinity columns), as well as epitope tags (e.g.,for detection, immunoprecipitation, or FACs analysis), such as myc, BSFbiotinylation target sequences of the bacterial enzyme BirA, flu tags,lacZ, GST, and Strep tags I and II. Nucleic acids encoding such tagmolecules are commercially available.

Stability sequences can be added to the fusion molecule to protect themolecule from degradation (e.g., by a protease). Suitable stabilitysequences include, but are not limited to, glycine moleculesincorporated after the initiation methionine (e.g., MG or MGG) toprotect the fusion molecule from ubiquitination; two pralinesincorporated at the C-terminus (conferring protection againstcarboxypeptidase action), and the like.

In some aspects, the fusion molecule can include a linking or tetheringsequence between insertion and acceptor sequences or between insertionor acceptor sequences and other domain sequences. For example, usefullinkers include glycine polymers, glycine-serine polymers,glycine-alanine polymers, alanine-serine polymers, alanine polymers, andother flexible linkers as are known in the art (see, e.g., Huston, etal, 1988, Proc. Natl. Acad. Sci USA 85: 4879; U.S. Pat. No. 5,091,513).

These additional sequences can be included to optimize the properties ofthe fusion molecules described herein.

Generating Fusion Molecules Comprising Domain Insertions

In one aspect, libraries in which an insertion sequence has beenrandomly Inserted into art acceptor sequence are constructed.Preferably, such libraries are generated by randomly inserting a nucleicacid fragment encoding an Insertion sequence into a nucleic acidfragment encoding an acceptor sequence.

All existing methods for random insertion can be categorized into one oftwo strategies: insertion via transposons and insertion after a randomdouble stranded break in DNA using one or a combination of nucleases. Avariety of transposons have been used to deliver short, in-frameinsertions of 4-93 amino acids (e.g., Hayes and Hallet, 2000, TrendsMicrobiol. 8: 571-7; Manoil and Traxier, 2000, Methods 20: 55-61).However, although transposons are an efficient method for delivering aninsertion, insertion methods are preferred which create libraries withdirect insertions, deletions at the insertion site, or variability inthe amount deletions or tandem duplication or variability in thedistribution of direct insertions, deletions and tandem duplications.

Random insertion using nuclease treatment, on the other hand, can createsuch libraries. These methods typically are used for the insertion ofshort sequences into a target gene during linker scanning mutagenesis.These methods generally differ in the strategy used to produce a random,double-strand break. In supercoiled plasmid DNA containing the gene tobe inserted.

A number of different strategies can be used to create the fusionmolecules of the instant invention. These include, but are not limitedto: (a) limited digestion with DNaseI in the presence of Mn²⁺ to producea single double stranded break (Keffron, et al. 1978, Proc. Natl. Acad.Sci. USA 75: 6012-6016); (b) limited digestion with DNaseI in thepresence of Mg²⁺ to produce a single nick followed by S1 nucleasetreatment to cleave opposite the nick (Dykxhoorn, et al., 1997, NucleicAcids Res. 25: 4209-18); (c) limited digestion with DNaseI with Mg²⁺under conditions for nick translation to take place, followed by S1nuclease treatment to cleave opposite the nick; and (d) partialapurination with formic acid and exonuclease III, which introduces asingle strand gap at the apurinic site, followed by S1 nucleasetreatment to cleave opposite the gap (Luckow, et al., 1987, NucleicAcids Res. 15: 417-429 (1987) summarized in FIG. 2B. In method (b), thelocation of the double strand break is determined by the location of theDNaseI nicking whereas in method (c) the location of the double strandbreak is determined by how far nick translation has progressed. Inaddition to digestion by nucleases (e.g., DNAse, S1, exonucleases,restriction endonucleases and the like), other methods for introducingbreaks in sequences can be used. For example, mechanical shearing,chemical treatment, and/or radiation can be used. Generally, the methodfor introducing breaks is not intended to be limiting.

In a particularly preferred aspect, libraries of fusion molecules aregenerated using incremental truncation (see, Patent Application byOstermeier, “Incrementally Truncated Nucleic Acids and Methods of Makingthe Same”, U.S. patent application Ser. No. 09/718465). As shown in FIG.2C, a key step in the creation of these libraries is the digestion ofthe gene fragments with a 3′ to 5′ exonuclease such as Exonuclease III(Exo III) under conditions (e.g., low temperature or in the presence ofNaCl) such that the digestion rate is controlled to ˜10 bases/minute orless. During Exo III digestion, small aliquots are removed frequentlyand quenched by addition to a low pH, high salt buffer. Blunt ends areprepared by treatment with a single-strand nuclease and a DNA polymerasefollowed by unimolecular ligation to recyclize the vector. As Exo IIIdigests DNA at a substantially uniform and synchronous rate (Wu, et al.,1976, Biochemistry 15: 734-740), this allows the creation of a librarycomprising every possible one base pair deletion of a gene or genefragment.

Constructing a Target Vector Comprising Acceptor Sequences

In one aspect, construction of a library comprises the initial step ofconstructing and testing a target vector, i.e., a vector comprising anucleic acid encoding an acceptor sequence. For example, a gene or genefragment which encodes a polypeptide is cloned into a vector, such as aplasmid. Preferably, the polypeptide exists in a state at least undercertain conditions, i.e., comprises an activity, can bind a molecule,exist in a conformation, emit light, transfer electrons, catalyze asubstrate, etc. under those conditions.

Preferably, the plasmid comprises a reporter sequence for monitoring theefficacy of the cloning process. Suitable reporter genes include anygene that expresses a detectable gene product which may be RNA orprotein. Examples of reporter genes, include, but are not limited to:CAT (chloramphenicol acetyl transferase); luciferase, and other enzymedetection systems, such as β-galactosidase, firefly luciferase,bacterial luciferase, phycobiliproteins (e.g., phycoerythrin); GFP;alkaline phosphatase; and genes encoding proteins conferringdrug/antibiotic resistance, or which encode proteins required tocomplement an auxotrophic phenotype. Other useful reporter genes encodecell surface proteins for which antibodies or ligands are available.Expression of the reporter gene allows cells to be detected or affinitypurified by the presence of the surface protein.

The reporter gene also may be a fusion gene that includes a desiredtranscriptional regulatory sequence, for example, to select for a fusionmolecule whose switching functions include the ability to modulatetranscription.

Generation of Insertion Sequences

Nucleic acids encoding polypeptide insertion sequences can be obtainedvia a number of routes, including, but not limited to one or more of:amplification (e.g., using primers which flank a nucleic acid sequenceencoding a domain of interest), reverse transcription, cloning, andchemical synthesis.

In one aspect, a nucleic acid can be amplified using primers designed toprovide convenient restriction sites or promoter sequences for furthercloning steps. This nucleic acid can be cloned into a vector anddigested with restriction endonucleases as in FIG. 2A to produce thedesired insertion sequence.

Construction of Random Insertion Libraries

In one aspect, a target vector comprising the nucleic acid encoding theacceptor polypeptide is randomly linearised (see, FIGS. 2B and 2G). Avariety of different nucleases and digestion schemes can be used. Forexample, the vector may be exposed to DNase/Mn²⁺ digestion followed bypolymerase/ligase repair; S1 nuclease digestion followed bypolymerase/ligase repair; and S1 nuclease digestion which is notrepaired. The three schemes differ in (a) the methods used to create therandom double-stranded break in the target plasmid and (b) whether ornot the nucleic acid (e.g., DNA) is repaired by polymerase/ligasetreatment, or other methods. However, it should be obvious to those ofskill in the art that any method of introducing breaks into a DNAmolecule ears be used (e.g., such as digestion by suing bean nucleases,endonucleases, restriction enzymes, exposure to chemical agents,irradiation, and/or mechanical shearing) and that the methods ofintroducing breaks described above are not intended to be limiting.

Preferably, digestion is controlled such that a significant fraction ofDNA is undigested in order maximize the amount of linear DNA that onlyhas one double strand break (see, e.g., Example 1, Table 2). Keyfeatures for optimizing DNase I digestion include the use of Mg²⁺ freeDNaseI (Roche Molecular Biochemicals), a digestion temperature of 22° C.and 1 mM Mn²⁺ instead of Mg²⁺ to increase the ratio of double strandbreaks to nicks (see, e.g., as described in Campbell and Jackson, 1980,J. Biol. Chem 255: 3726-35).

The DNA can be repaired rising methods known in the art, for example,using T4 DNA ligase and T4 DNA polymerase (see, e.g., Graf andSchachman, 1996, Proc. Natl. Acad. Sci, USA 93: 11591-11596) anddephosphorylated. Ligation with nucleic acids encoding the insert isperformed and the collection of nucleic acids (e.g., library member).

Incremental truncation libraries can be used to examine all possibleinsertion points within a given region of an acceptor molecule (see,FIG. 2C). Incremental truncation used within the context of the presentinvention is a combinatorial solution to identifying active, bisectedproteins that would be difficult, to predict a priori. Libraries can berecombined in vitro by methods such as DNA shuffling (Stemmer, 1994,Proc. Natl. Acad. Sci. USA 91: 10747-10751) to explore new areas ofsequence space (see, e.g., Lutz, et al., 2001, Proc. Natl. Acad. Sci.USA 98: 11248-11253).

Preferably, random insertion libraries according to the inventioncomprise at least about 10⁴-10⁸ library members. More preferably,insertion libraries comprise at least two times the number of base pairsin a target nucleic acid (e.g., a nucleic acid comprising acceptor DNAand other vector sequences). More preferably, a library comprises one ormore of: deletions at the insertion site and duplications at theinsertion site, as well as direct insertions with neither duplicationsnor deletions. Generally, library members may comprise small deletionsor tandem duplications on the order of at least about 1-20 bases;however, larger duplications or deletions on the order of about half thelength of a gene also may be tolerated and/or desirable.

Evaluation of Insertion Libraries: Identification of Fusion Molecules

In one aspect, transformants are selected which express a reporter geneincluded in the target vector, such as a drug resistance gene toinitially screen for fusion molecules. Alternatively, or additionally,transformants can be selected in which the state of the insertionsequence is coupled to the state of the acceptor sequence (see, e.g.,FIG. 2D). Thus, in one aspect, the existence of each state is assayedfor, as is the dependence of each state on existence of one or moreother states. States may be assayed for simultaneously, or sequentially,in the same host cell or in clones of host cells. Fusion molecules alsocan be Isolated from host cells (or clones thereof) and their states canbe assayed for in vitro.

For example, in one aspect, the enzymatic activity of an insertionsequence or acceptor sequence is assayed for at the same that thebinding activity of the respective other portion of the fusion isevaluated (see, e.g., as described further in Example 1, and Table 2) toidentity fusion molecules in which enzymatic activity is dependent onbinding activity.

In another aspect, fusion molecules are screened for which bind to amolecule, such as a bio-effective molecule (e.g., a drug, therapeuticagent, toxic agent, agent for affecting cellular physiology). The boundfusion molecule is exposed to a cell, and the ability of the fusionmolecule to be localized intracellularly is determined. Preferably,release of the bio-effective molecule in response to intracellularlocalization also is determined.

For example, a cell can be transiently permeabilized (e.g., by exposureto a chemical agent such as Ca²⁺ or by electroporation) and exposed to afusion molecule associated with the bio-effective molecule (e.g., boundto the bio-effective molecule), allowing the fusion molecule and boundmolecule to gain entry into the cell. The ability of the fusion moleculeto localize to an intracellular compartment (e.g., to the endoplasmicreticulum, to a lysosomal compartment, nucleus, etc.) along with thebio-effective molecule can be monitored through the presence of a label(e.g., such as a fluorescent label or radioactive label) on the fusionmolecule, bio-effective molecule, or both. The label can be conjugatedto the fusion molecule and/or the bio-effective molecule using routinechemical methods known in the art. A label also may be provided as partof an additional domain of the fusion molecule. For example, the fusionmolecule can comprise a GFP polypeptide or modified form thereof. Thelocalization of the label (and hence the fusion molecule and/orbio-effective molecule) can be determined using light microscopy.Release of the bio-effective molecule can be monitored by lysing thecell, immunoprecipitating the fusion molecule, and detecting the amountof labeled bio-effective molecule in the precipitated fraction.

In one aspect, the cell need not be permeabilized to allow entry of thefusion molecule because the fusion molecule comprises signal sequencethat enables the fusion molecule to traverse the cell membrane.Intracellular transport of the bio-effective molecule can be monitoredby labeling the bio-effective molecule and examining its localizationusing light microscopy, FACs analysis, or other methods routine in theart.

In another aspect, insertion libraries are screened for fusion moleculeswhich comprise an insertion sequence or acceptor sequence whichassociates with a bio-effective molecule and which releases thebio-effective molecule when the respective other portion of the fusionbinds to a cellular marker of a pathological condition. Thus, In oneaspect, fusion molecules associated with a bio-effective molecule arecontacted with cells expressing such a marker and the ability of thefusion molecules to specifically bind to the cell is assayed for, aswell as the ability of the fusion molecule to release the bio-effectivemolecule in response to such binding. For example, as above, either, orboth, the fusion molecule and the bio-effective molecule can be labeledand the localization of the molecules determined. The action of thebio-effective molecule also can be monitored (e.g., the effect of thebio-effective molecule on the cell can be monitored).

In a preferred aspect, the insertion library comprises members in whichthe insertion or acceptor sequence comprises the human serum transferrin(HST) transport domain while the respective other portion of the fusioncomprises a binding domain for binding to an anti-cancer drag. In onepreferred aspect, the binding domain comprises the methotrexate-bindingdomain of the dihydrofolate reductase polypeptide (DHFR). At least twomethods for the identification of fusions with fee desired activity canbe used. In the first, a DHFR-HST library is displayed on the surface ofphage and panned against methotrexate immobilized on a solid phase suchas agarose. Fusions are selected for which bind the drag in the presenceof iron at physiological pH (7.4), but which release methotrexate whenHST releases its iron in a mildly acidic wash. After each round ofselection, the library will be sampled and DHFR activity atphysiological and acidic pH will be measured in order to evaluate fusionmolecules selected.

The second strategy takes advantage of selective inhibition of bacterialDHFR by the antibacterial drug trimethoprim. E. coli cannot grow in thepresence of trimethoprim unless the bacteria is expressing a functionalmammalian DHFR. Therefore, in a first step, a non-phage display libraryof DHFR-HST fusions is expressed in E. coli and those fusions thatexhibit DHFR activity is selected by growth on plates at physiologicalpH containing trimethoprim. Assuming that DHFR activity correlates withmethotrexate binding and that conformational changes in the DHFR-HSTfusion that disrupt trimethoprim binding also disrupt methotrexatebinding, those colonies selected in the first step are screened for nogrowth on plates at acidic pH containing trimethoprim in order toidentify fusions with the ability to release methotrexate at acidic pH.

In still another aspect, insertion libraries are screened for fusionmolecules which can switch from a non-toxic state to a toxic state uponbinding of the insertion sequence or acceptor sequence to a cellularmarker of a pathology. As above, fusion molecules can be selected whichspecifically bind to cells expressing the marker and fee affect of thefusion molecules on cell death can be assayed for. Cell death can bemonitored rising methods routine in the art, including, but not limitedto: staining cells with vital dyes, detecting spectral propertiescharacteristic of dead or dying cells, evaluating the morphology of thecells, examining DNA fragmentation, detecting the presence of proteinsassociated with cell death, and the like. Cell death also can beevaluated by determining the LD₅₀ or LC₅₀ of the fusion molecule.

In a further aspect, the insertion library is screened for fusionmolecules which comprises a molecular switch for controlling a cellularpathway. Preferably, the states of the insertion sequence and acceptorsequence in the fusion molecules are coupled and responsive to a signalsuch that in the presence of the signal, the state of either theinsertion sequence or the acceptor sequence modulates the activity orexpression of a molecular pathway molecule in a cell. A signal can bethe presence, absence, or level, of an exogenous or endogenous bindingmolecule to which either the insertion sequence or acceptor sequencebinds, or can be a condition (e.g., chemical, optical, electrical, etc.)in an environment to which the fusion molecule is exposed. The abilityof the fusion molecule to control a pathway can be monitored byexamining the expression and/or activity of pathway molecules which actdownstream of a pathway molecule whose expression and/or activity isbeing modulated.

In another aspect, fusion molecules are selected in which either theinsertion sequence or acceptor sequence binds to a nucleic acidmolecule. For example, the ability of fusion molecules to bind to anucleic acid immobilized on a solid phase can be monitored (e.g.,membrane, chip, wafer, particle, slide, column, microbead, microsphere,capillary, and the like). Preferably, fusion molecules are selected inwhich nucleic acid binding activity is coupled to a change in state ofthe respective other sequence of the fusion molecule. For example,nucleic acid binding activity can be coupled to the binding activity ofanother portion of the fusion molecule, catalysis by the other portion,the light emitting function of the other portion, electron transferringability of the other portion, ability of the other portion to changeconformation, and the like. Preferably, nucleic acid binding activity iscoupled to the response of the fusion molecule to a signal.

Nucleic acid binding activity also can be monitored by evaluating theactivity of a target nucleic acid sequence to which the fusion moleculebinds. For example, in one aspect, the fusion molecule binds to anucleic acid regulatory sequence which modulates the activity (e.g.,transcription, translation, replication, recombination, supercoiling) ofanother nucleic acid molecule to which the regulatory sequence isoperably linked. The nucleic acid regulatory molecule and its regulatedsequence can be provided as part of a nucleic acid molecule encoding thefusion molecule or can be provided as part of separate molecule(s). Thenucleic acid binding activity can be monitored in vitro or in vivo. Theability of fusion molecules to bind to a nucleic acid can also bedetermined in viva using one-hybrid or two-hybrid systems (for example,see, Hu, et al, 2000, Methods 20: 80-94.

In certain aspects, fusion molecules are selected which bind to a knownregulatory sequence or a sequence naturally found in a cell. In otheraspects, a sequence which is not known to be a regulatory sequence in acell is selected for. Preferably, such a sequence hinds to the fusionmolecule and modulates the activity of another nucleic acid (in cis orin trans). Thus, the fusion molecule can be used to select for novelnucleic acid regulatory sequences. Preferably, the fusion moleculemodulates the regulatory activity of the nucleic acid molecule inresponse to a signal, as described above.

In still a further aspect, the insertion library is screened for fusionmolecules which are sensor molecules. Preferably, fusion molecules arescreened for in which either the insertion sequence or acceptor sequencebinds to a target molecule and wherein the respective other portion ofthe fusion molecule generates a signal in response to binding. Signalscan include: emission of light, transfer of electrons, catalysis of asubstrate, binding to a detectable molecule, and the like. To assay forsuch fusions, members of the library can be screened in the presence ofthe target molecule (e.g., In solution, or immobilized on a solidsupport) for the production of the signal.

Evaluation of Structure: State Relationships in Fusion Molecules

Preferably, random library members having desired states are sequencedto precisely identify the sequence of the fusions at the insertion site.More preferably, all library members having desired states aresequenced. Sequence information can be correlated with the ability ofdifferent portions of the fusion molecule to maintain one or more statesand to respond to one or more signals. A plurality of active insertionpoints, and preferably, all possible insertion points, can be mappedonto a crystal structure of the acceptor sequence (e.g., such as anacceptor polypeptide). Sites of insertion that produce allostericcontrol can be compared to sites in tire acceptor molecule predicted tobe allosterically linked to a signaling molecule (e.g., such as abinding molecule or ligand) by comparisons of the structures of acceptormolecule in the presence or absence of the signaling molecule (see,e.g., Starzyk, et al., 1989, Biochemistry 28: 8479-8484).

In another aspect, non-functional fusion molecules also axe sequenced todetermine structures which are not appropriate to maintain particularstates and/or respond to signals.

In a further aspect, fusion molecules are mutagenized to identifymolecular switches with optimal properties. Preferably, the sequence ofsuch molecules also are determined. In one aspect, “first roundswitches” are identified by screening a library of domain insertions andoptimized to select for “second round switches” with improvedproperties. For example, combinatorial (e.g., error-prone PCR, DNAshuffling, etc) and/or rational methods can be used to select forswitches with increased activity, stability, and/or improved switchingcapacity (e.g., ability to respond to a wider or narrow range ofsignal). Preferably, second round switches are also sequenced toidentify sequence alterations associated with improved properties.

Conditional Heterodimerization

Many proteins can have their peptide backbone cut by proteolytic orgenetic means, yet the two fragments can associate to make an activeheterodimer. This phenomenon of “monomer to heterodimer conversion” isreferred to as protein fragment complementation. However, there are manylocations where such a conversion it is not feasible, presumably due toinefficient assembly or improper folding of the fragments. This can beovercome by fusion of the fragments to dimerization domains tofacilitate correct assembly. Such “assisted protein reassembly” has beenshown for a few proteins (Pelletier, et al., 1998, Proc. Natl. Acad. SciUSA 95: 12141-12146; Spencer, et al., 1993, Science 262: 1019-24;Michnick, et al., 2000, Methods Enzmol 328: 208-30; Remy and Michnick,1999, Proc. Natl. Acad. Sci. USA 96: 5394-5399, 7620; Remy, et al.,1999, Science 283: 990-993; Ghosh, et al., 2000, J. Am. Chem. Soc. 122:5658; Johnson and Varshavsky, 1994, Proc. Natl. Acad. Sci USA 91,10340-10344; Karimova, et al., 1997, Proc. Natl. Acad. Sci. USA 94:8405-8410; Rossi, et al., 2000, Methods Enzymol. 328: 231-51). However,thus far, such methods have been used exclusively in two-hybrid systemto evaluate protein-protein interactions (Remy and Micknick, 1999,supra; Arndt, et al., 2000, J. Mol. Biol. 295: 627-39; Pelletier, etal., 1999, Nat Biotechnol 17: 683-90; Mossner, et al., 2001, J. Mol.Biol. 308: 115-22) and have not been exploited to generate molecularswitches.

The invention provides a pair of fusion molecules comprising a firstportion and second portion. The first and second portions represent thefragments of a bisected polypeptide which cannot function or exist in aparticular state unless both portions are rought into sufficientproximity. Preferably, each portion is fused to an oligomerization omain(see, e.g., FIG. 1B, FIGS. 5A-C, and Example 2 below) thereby generatinga pair of fusion molecules. Unlike the protein fragment complementationsystems described in the prior art, the fusion molecules according tothe invention oligomerize only is the presence of a signal, providing ameans to switch ON the activity/state of the polypeptide in the presenceof the signal. Suitable signals include any described above for domaininsertion fusion molecules.

Suitable oligomerization motifs include, but are not limited to,dimerization motifs such as the LexA dimerization domain (Golemis andBrent, 1992, Mol. Cell Biol. 12; 3006), lambda cI dimerization domain,leucine zipper dimerization domains (e.g., such as from GCN4 leucinezippers, antiparallel leucine zippers, p21, and the like), rasGTPase/ras-binding domain, FADD/FAS dimerization domains, BGF receptordimerization domains, the FKBP/FRAP dimerization domains, thetetramerization domain of p53, and the tetramerization domain ofBCR-ABL. In addition, the art also provides a variety of techniques foridentifying other naturally occurring oligomerization domains, as wellas oligomerization domains derived from mutant or artificial sequences(see, e.g., Zeng et al., 1997, Gene 185: 245).

In a preferred aspect, leucine zippers are used as dimerization domainsto assemble fragments of a polypeptide. Each domain of a leucine zipperis relatively simple, comprising an approximately 30 amino acid helix.Further, depending on their sequence, leucine zippers can dimerize in aparallel or antiparallel configuration, thus offering two distinctgeometries for re-assembly of an active polypeptide. Both parallel andantiparallel leucine zippers have been shown to assist the reassembly offragments of proteins. Because much is known about the interactions thatstabilize dimerization, zippers of different affinity are readilyavailable. Finally, leucine zippers have been shown to be expressed wellin E. coli.

In one-preferred aspect, oligomerization occurs on binding of theoligomerization domains to a small molecule, such as a CID. A CID is asynthetic ligand having two binding surfaces that facilitate thedimerization of domains fused to target proteins (see, e.g., Spencer, etal., 1993, Science 262: 1019-24; Rivera, et al., 1998, Methods 14:421-9). CIDs have been used to facilitate the dimerization of domainsfused to target proteins. CIDs also have been used to initiate signalingpathways by dimerizing receptors on the cell surface, to translocatecytosolic proteins to the plasma membrane, to import and export proteinsfrom the nucleus, to induce apoptosis, and to regulate genetranscription (Farrar, et al., 2000, supra; Bishop, et al., 2000, Annu.Rev. Biophys. Biomol. Struct. 29: 577-606. However, CIDs reported in theart have not been used as switches to activate previously inactiveproteins in cells.

Suitable CIDs for use in the present invention include, but are notlimited to: the immunosupressant FK506 (Spencer, et al., 1993, supra);coumermycin (which induces dimerization of GyrB-containing fusionproteins) (see, Farrar, et al., 2000, Methods Enzymol 327: 421-9), andrapamycin. Novel CID's can be screened for using combinatorial librariesto identify molecules capable of inducing oligomerization ofoligomerizing domains.

Types of proteins which can be bisected generally can include any of thedomains described above as suitable for insertion sequences or acceptorsequences. In one aspect, bisected molecules include, but are notlimited to: dihydrofolate reductase (DKFR) (Pelletier, et al., 1998,Proc. Natl. Acad. Sci. USA 95: 12141-12346; Remy, et al., 1999, Proc.Natl. Acad. Sci. USA 96: 5394-5399; Remy, et al., 1009, Science 283:990-993); E. coli glycinamide ribonucleotide transformylase (PurN)(Michnick, et al., 2000, supra); green fluorescent protein (Ghosh, etal., 2000, J. Am. Chem. Soc. 122: 5658), ubiquitin (Johnson andVarshavsky, 1994, Proc. Natl. Acad. Sci. USA 91: 10340-10344; Karimova,et al., 1998, Proc. Natl. Acad. Sci. USA 95: 5752-6), β-galactosidase(Rossi, et al., 1997, Proc. Natl. Acad. Sci. USA 94: 8405-8410; Rossi,et al, 2000, Methods Enzymol 328: 231-51); aminoglycoside and hygromycinB phosphotransferases (Michnick, et al., 2000, supra), as these havebeen shown to be tolerant of bisections.

Fusion molecules additionally may comprise flexible linkers, stabilizingsequences, affinity sequences, and the like, as described above.

In contrast to reassembled proteins described in the art, theconditional heterodimers of the invention may include duplicatedresidues and/or deletions at the site of bisection. As shown in FIG. 7B,in one aspect, libraries comprising the heterodimers may have small tolarge duplications and/or deletions in both nucleic acid fragmentsencoding the respective portions of the bisected polypeptide, increasingthe diversity of molecules which may be evaluated for switchingfunction. Further, unlike reassembled proteins described in the art,linker sequences are not required between the dimerization domain andthe bisected portion of the polypeptide. Therefore, in one aspect, theinvention provides a fusion molecule comprising a portion of a bisectedpolypeptide fused to an oligomerization domain, wherein the fusionmolecule does not comprise a linker sequence and the oligomerizationdomain is responsive to a signal. Preferably, the response of theoligomerization domain to the signal brings respective portions of thebisected polypeptide together.

In another aspect, the invention provides a pair of fusion moleculeswhich each comprise respective portions of a bisected polypeptide fusedto oligomerization domains, wherein the respective portions of thebisected polypeptide are encoded by nucleic acids comprising aduplication or deletion at the bisection site.

Generation of Conditional Heterodimers

The strategy for generating pairs of fusion molecules for formingconditional heterodimers is illustrated in FIGS. 6A-B. In the exampleshown in the Figures, a polypeptide comprising an activity (e.g., suchas an enzymatic activity) is systematically bisected by fragmenting agene encoding the polypeptide to generate a plurality of bisectedpolypeptides. Preferably, all possible bisections are represented. Insubsequent, or the same cloning steps, nucleic acids encodingoligomerization sequences are ligated in frame to the nucleic acidsencoding the plurality of bisected polypeptides. Pairs of fusionmolecules so generated are screened for those which are able to dimerize(e.g., restoring the activity of the bisected polypeptides).

In one aspect, incremental truncation is used to engineer a conditionalheterodimer. In the example for implementing this approach, shown inFIGS. 6A-B, two overlapping fragments of a gene encoding a polypeptidewhose state is to be switched are closed into vectors. Incrementaltruncation libraries from the 3′ end of the 5′ fragment and the 5′ endof the 3′ fragment are prepared using time-dependent exonucleasedigestion (Ostermeier, et al., 1999, Proc. Natl. Acad. Sci. USA 96:3562-3567) or α-phosphothioate nucleotide incorporation (Lutz, et al.,2001, Nucleic Acids Res, 29: e16) to generate linear fragments.Preferably, as with domain insertion libraries, these libraries comprisedeletions and/or duplications at the insertion site.

To avoid the possibility that individual fragments are active on theirown, the starting fragments preferably are designed such that they lackessential residues for functionality (e.g., such as residues at theN-terminal encoding portion or C-terminal encoding portion of thefragments). After truncation, vectors are circularized such that the 3′truncated fragment is fused to stop-codons in all three reading framesand the 5′ truncation is fused to an ATG start codon. Separate librariesof 5′ and 3′ digested fragments are introduced into E. coli atconcentrations that will maximize co-transformation of the 5′ and 3′fragments, i.e., providing the potential to detect pairs of fusionmolecules which dimerize in response to a signal. Nucleic acids encodingoligomerization domains (e.g., such as dimerization domains) can belinked to the fragments before or after or during the creation of thetruncation libraries (e.g., by oligo assembly or by PCR). Preferably,the oligomerization domains are responsive to a signal. The ability ofcells to recover polypeptide activity in the presence or absence of theoligormerization domain, and in the presence or absence of signal, ismonitored.

Cells exhibiting protein activity in the presence of signal areidentified and the vectors expressing the respective halves of thepolypeptide are sequenced. In one aspect, pairs of fusion moleculesexhibiting the highest degree of activity are selected as targets fordirected evolution. For example, gene fragments can be amplified byerror-prone PCR (Caldwell and Joyce, 1995, in PCR Primer: A LaboratoryManual, Cold Spring Harbor-Laboratory Press, Plainview, N.Y.) such thaton average each DNA molecule has one missense mutation. Such 5′ and 3′gene fragments are again co-transformed and cells are selected whichexpress the same or higher levels of activity. Preferably, cells thatexpress higher levels of activity are identified (e.g., at least about2-fold higher activity). Rescued constructs are sequenced to identifythe nature of the mutation and to verily that mutations are not creatingfragments whose encoded, polypeptides oligomerize even in the absence ofan oligomerization domain.

In one aspect, after identifying pairs of fusion molecules whoseactivity can be restored through oligomerization, the oligomerizationdomains of these pairs are exchanged for oligomerization domains whichare responsive to a signal (e.g., where original domains where notresponsive to a signal) or which respond to a different signal from onerecognized by domains used to create the original fusion molecules.

Expression Vectors for Expressing Fusion Molecules

Identification of desired fusion molecules, whether domain insertions,or conditional heterodimers, can be facilitated by the use of expressionvectors in creating the libraries described above. Such expressionvectors additionally can be useful for generating large amounts offusion molecules (e.g., for delivery to a cell, or organism, for use invitro or in vivo).

Thus, in one aspect, library members comprise regulatory sequences(e.g., such as promoter sequences) which can be either constitutivelyactive or inducible which are operatively linked to acceptor sequencescomprising insertion sequences. Regulatory sequences can comprisepromoters and/or enhancer regions from a single gene or can combineregulatory elements of more than one gene. In a preferred embodiment,the regulatory sequences comprise strong promoters which allow highexpression in cells, particularly in mammalian cells. For example, thepromoter can comprise a CMV promoter and/or a Tet regulatory element.

Library members also can comprise promoters to facilitate in vitrotranslation (e.g., T7, T4, or SP6 promoters). Such constructs can beused to produce amounts of fusion molecules in sufficient quantity toverify initial screening results (e.g., the ability of the molecules tofunction as molecular switches).

The expression vectors can be self-replicating extrachromosomal vectorsand/or vectors which integrate into a host genome. In one aspect, theexpression vectors are designed to have at least two replicationsystems, allowing them to be replicated and/or expressed and/orintegrated in more than one host cell (e.g., a prokaryotic, yeast,insect, and/or mammalian cells). For example, the expression vectors canbe replicated and maintained in a prokaryotic cell and then transferred(e.g., by transaction, transformation, electroporation, microinjection,cell fusion, and the like) to a mammalian cell.

The expression vectors can include sequences which facilitateintegration into a host genome (e.g., such as a mammalian cell). Forexample, the expression vector can comprise two homologous sequencesflanking the nucleic acid sequence encoding the fusion molecule,facilitating insertion of the nucleic acid expressing the fusionmolecule into the host genome through recombination between the flankingsequences and sequences in the host genome. Sequences such as lox-cresites also can be provided for tissue-specific inversion of the fusionmolecule nucleic acid with respect to a regulator sequence to which thefusion molecule nucleic acid is operably linked.

Integration into the host genome may be monitored by screening for theexpression of a reporter sequence included in the expression vector, bythe expression of the unique fusion molecule (e.g., by monitoringtranscription via Northern Blot analysis or translation by animmunoassay), and/or by the presence of the switching activity in thecell.

Host Cells for Expressing Fusion Molecules

Fusion molecules according to the invention, can be expressed in avariety of host cell, including, but not limited to: prokaryotic cells(e.g., E. coli, Staphylococcus sp., Bacillus sp.); yeast cells (e.g.,Saccharomyces sp.); insect cells; nematode cells; plant cells; amphibiancells (e.g., Xenopus); fish cells (e.g., zebrafish cells); avian cells;and mammalian cells (e.g., human cells, mouse cells, mammalian celllines, primary cultured mammalian cells, such as from dissectedtissues).

The molecules can be expressed in host cells isolated from an organism,host cells which are part of an organism, or host cells which areintroduced into an organism. In one aspect, fusion molecules areexpressed in host cells in vitro, e.g., in culture. In another aspect,fusion molecules are expressed in a transgenic organism. (e.g., atransgenic mouse, rat, rabbit, pig, primate, etc.) that comprisessomatic and/or germlme cells comprising nucleic acids encoding thefusion molecules.

Fusion molecule also can be introduced into cells in vitro, and thecells (e.g., such as stem cells, hematopoietic cells, lymphocytes, andthe like) can be introduced into the host organism. The cells may beheterologous or autologous with respect to the host organism. Forexample, cells can be obtained from the host organism, fusion moleculesintroduced into ins cells in vitro, and then reintroduced into the hostorganism.

Methods of Using Molecular Switches

In one aspect, the invention provides a method for using a molecularswitch to modulate a cellular activity. The cellular activity caninclude an enzyme activity, the activity of one or more cellular pathwaymolecules, the transduction of a signal, and the like. Modulation maydirect, e.g., the switch itself may alter the activity, or indirect,e.g., the switch may function by delivering a bio-effective molecule tothe cell which itself modulates the activity. Modulation can occur invitro (e.g., in cell culture or an a cell extract) or is vivo (e.g.,such as in a transgenic organism). Molecular switches comprising fusionpolypeptides also can be administered to a cell by delivering suchmolecules systemically (e.g., through intravenous, intramuscular, orintraperitoneal injections, or through oral administration of either thepolypeptides themselves or nucleic acids encoding the polypeptides) orlocally (e.g., via injection into a tumor or into an open surgicalfield, or through a catheter or other medical access device, or viatopical administration).

In one aspect, molecular switches are used to conditionally modulate anenzymatic activity in a cell. For example, a switch molecule can beintroduced into a cell mat comprises an insertion sequence or acceptorsequence which provides the enzymatic activity. Catalysis by theinsertion or acceptor sequence is coupled to the response of therespective other portion of the fusion molecule to a signal, such asbinding of the other portion to a molecule (e.g., such as an agentadministered to the cell or a naturally occurring small molecule),exposure of the cell to particular chemical conditions (e.g., such aspH), electrical conditions (e.g., potential differences), opticalconditions (e.g., exposure of the cell to light of specificwavelengths), magnetic conditions and the like.

In another aspect, a molecular switch is provided which modulates theactivity or expression of a molecular pathway molecule in a cell. FIG.3B shows an example of a switch molecule comprising a pathway moleculewinch is conditionally active in the presence of a signal (schematicallyillustrated as in the Figure). The switch molecule is used to alter acell signaling pathway, e.g., altering the expression and/or activity ofdownstream pathway molecules (taming such molecules ON or OFF, oraltering the level of expression and/or activity of snob molecules). Indoing so, the switch molecule can be used to regulate fate of one ormore cells. Similarly, the molecular switches according to the inventioncan be used to control metabolic pathways, e.g., providing a fusionmolecule which provides an enzymatic activity coupled to the binding ofa small molecule, or response to some other signal (see, as shown inFIG. 3E). Preferably, modulation of the enzyme activity in response tothe signal, in turn, modulates the expression and/or activity ofmolecules downstream in the metabolic pathway.

More preferably, the slates of the fusion molecules are coupled to asignal, such as the presence of an exogenous or endogenous bindingmolecules to which either the insertion sequence or acceptor sequencebinds. The ability of the fusion molecule to control a pathway can bemonitored by examining the expression, and/or activity of pathwaymolecules which act downstream of a pathway molecule whose expressionand/or activity is being modulated/controlled by the fusion molecule.Preferably, control of the pathway is coupled to the presence of thesignal, e.g., binding of the fusion molecule to the exogenous orendogenous binding molecule, the presence of particular electrical orchemical properties of a cell, the presence or absence of particularwavelength(s) of light, and the like.

Pathways of interest include the phosphatidylinositol-specificphospholipase pathway, which is normally involved with hydrolysis ofphosphatidylinositol-4,5-bisphosphate and which results in production ofthe secondary messengers inositol-1,4,5-trisphosphate anddiacylglycerol. Other pathways include, but are not limited to: a kinasepathway, a pathway involving a G Protein Coupled Receptor, aglucerebrosidase-mediated pathway, a cylin pathway, an anaerobic oraerobic metabolic pathway, a blood clotting pathway, and the like.

In still another aspect, a fusion molecule is provided which, delivers abio-effective molecule (e.g., a drug, therapeutic agent, diagnostic orimaging agent, and the like) to a cell. In one scenario, shown in FIG.3C, the fusion molecule comprises an insertion or acceptor sequencewhich binds to the bio-effective molecule, while the respective otherportion of the fusion binds to a cellular marker that is a signature ofa pathology, e.g., a small molecule, polypeptide, nucleic acid,metabolite, whose expression (presence or level) is associated with thepathology. Preferably, the fusion molecule releases the bio-effectivemolecule only in the presence of the marker of due pathology.

FIG. 3D shows an alternative method of transporting a bio-effectivemolecule. In tins aspect, the insertion sequence or acceptor sequencecomprises a transport sequence for transporting a bio-effective moleculebound to the fusion molecule intracellularly. Preferably, the insertionsequence and acceptor sequence are functionally coupled such that aconformational change in the transport sequence is coupled tointracellular release of the bio-effective agent. Successful deliverycan be monitored by measuring the effect of the bio-effective agent(e.g., its ability to mediate a drug action or therapeutic effect, or toimage a cell). More preferably, the conformation change occurs uponresponse of the respective other portion of the fusion to a signal(indicated schematically in the Figure as □), enabling conditionalintracellular transport of the bio-effective molecule. When thebio-effective agent is delivered to one or more cells in an organism,the effect of the agent on the physiological responses of the organismcan be monitored, e.g., by observing clinical or therapeutic endpointsas is routine in the art. Where the bio-effective molecule is an imagingmolecule, the localization of the bio-effective molecule in the organismcan be monitored by MRI, X-ray, angioplasty, and the like.

In one preferred aspect, the transport sequence comprises the humanserum tranferrin (HST) polypeptide (see. FIG. 4). HST mediates thetransport and uptake of iron into cells. Iron-saturated HST binds to thetransferrin receptors on cell surfaces and is internalized byendocytosis. In endosomes, the pH becomes mildly acidic causing therelease of iron and a concomitant conformational change in HST. Thetransferrin-receptor recycles to the surface, where HST is released andis tree to bind more iron. As tumor cells express high levels oftransferrin receptors, several strategies for the targeted delivery oftoxic proteins and chemotherapeuttc drugs using transferrin uptakepathway have been pursued (Barbas, et al., 1992, J. Biol. Chem. 267:9437-9442; Trowbridge and Domingo, 1981, Nature 294: 171-173). Aclinical trial has demonstrated that an HST/diphtheria toxin conjugatewas effective for the treatment of recurrent malignant brain tumors inhumans (see, e.g., Laske, et al., pb 1997, Nat. Med. 3: 1362-1368). HSThas been demonstrated to tolerate insertions of peptides while retainingbiological activity (see, e.g., Ali et al., 1999, J. Biol. Chem. 274:24066-24073).

Therefore, in one aspect, the insertion sequence or acceptor sequencecomprises an HST polypeptide or active portion thereof, while therespective other portion binds to a bio-effective molecule. The bindingsequenced-HST sequence functions like a “Trojan horse” for transportingthe bio-effective molecule into cells, A suitable binding sequenced cancomprise a dihydrofolate reductase (DHFR) which binds to the anti-cancerdrag, methotrexate.

As shown in FIG. 4, outside the cell, the transferrin domain of the‘Trojan horse’ fusion molecule binds iron and the binding domain hindsthe drag. The fusion interacts with the transferrin receptor and isendocytosed. A decrease in pH in the endosome causes a conformationalchange in the transferrin domain, resulting in a conformational changein the drug binding domains which occurs concomitant with drug release.The fusion is recycled back outside of the cell to repeat the cycleagain. Because HST has a long circulating half-life and can continuouslycycle in and out of the a cell, multiple drug deliveries are possibleusing this scheme. Delivery of methotrexate can be optimized byselecting for fusion molecules which bind to methotrexate at loweraffinities than natural DHFR, e.g., by in silica modeling or frommutagenesis studies (see, e.g., Miller and Benkovic, 1998, Chem. Biol.5: R105-R113).

In still another aspect, the invention provides a method for killingundesired cells, such as abnormally proliferating cells (e.g., cancercells) (see, e.g., FIG. 3E). For example, a fusion protein comprising aconditionally toxic molecule which targets to a cell having a pathologycan be administered a cell (or an organism comprising the cell).Preferably, the toxic state of the fusion protein is coupled to theresponse of the fusion protein to a signal, such as exposure to a markerof a pathology, causing the fusion protein to switch from a non-toxicstate to a toxic state when it encounters the cell comprising thepathology. In one aspect, the change in state from a toxic to anon-toxic or less toxic molecule is coupled to binding of the fusionprotein to the marker of the pathology.

In a farther aspect, a fusion molecule is provided for regulating anactivity of a nucleic acid regulatory sequence in vitro or in vivo.Activities which can be regulated include transcription, translation,replication, recombination, supercoiling, and the like. Preferably,fusion molecules are selected in which binding of the insertion sequenceor acceptor sequence of the fusion molecule to the nucleic acidregulatory sequence is coupled to the response of the respective othersequence of the fusion molecule to a signal. Such fusion molecules canbe used to create cells with conditional knockouts or knock-ins of agene product whose expression is mediated by the activity of the nucleicacid regulatory sequence to which the fusion molecule binds, e.g., byproviding or withdrawing the signal as appropriate. In one aspect, feesignal is a drug or therapeutic agent. In another aspect, the signal isa change in pH, a change in cellular potential, or a change in exposureof a cell (and/or organism) to light. For example, a probe fordelivering particular wavelengths of light can be used to provide ahighly localized, signal to a cell expressing a fusion molecule in vivo.

In still a further aspect, the fusion molecules according to theinvention comprise sensor molecules that can be used to detect targetanalytes in vitro or in vivo (see, FIG. 3G). Target analytes include,but are not limited to: small molecules, metabolites, lipids,glycoproteins, carbohydrates, amino acids, peptides, polypeptides,proteins, antigens, nucleotides, nucleic acids, cells, cell organelles,and small organisms (e.g., microorganisms such as bacteria, yeast,protests, and the like).

The fusion molecule can be exposed to a target molecule in solution orstably associated with a solid support that can be exposed to a samplesuspected of containing the target molecule. Alternatively, the fusionmolecule can be expressed in a cell, i.e., for detecting intercellularor extracellular targets (for example, where the fusion moleculecomprises an extracellular binding domain). Analyte present in thesample wilt bind to the fusion molecule, triggering production of asignal by the signaling portion of the molecule. Suitable signalingmolecules from which this portion can be obtained include moleculescapable of emitting light, e.g., such as GFP, or modified, or mutantforms thereof (e.g., EGFP, YFP, CFP, EYFP, ECFP, BFP, and the like).Other signaling molecules include electron transferring domains (e.g.,such that the electrical characteristics of the fusion molecule can bemonitored to provide a measure of target analyte), binding domains(e.g., domains capable of binding to a labeled molecule), and catalyticdomains (e.g., β-lactamase, luciferase, alkaline phosphatase, and thelike).

Signaling molecules which comprise catalytic domains can be detected bymonitoring changes in the level of a fluorescent substrate. For example,when the catalytic domain is obtained from β-lactamase, fluorescentsubstrates such as CCF2/FA and CCF2/AM can be used (see, e.g.,Zlokarnik, et al., Science 279: 84-88 (1998)).

In a further aspect, the invention provides a method for modulating acellular response by conditionally providing a pair of fusionpolypeptides to a cell to mediate the response. For example, the pair offusion polypeptides can comprise a binding activity, an enzymaticactivity, a signaling activity, a metabolic activity, and the like. Inone aspect, the pair of fusion polypeptides modulate transcription,translation, or replication of the cell and/or alters a cellularphenotype in response to a signal.

Preferably, each member of the pair comprises a portion of a polypeptidefused to an oligomerization domain. Neither portion by itself canfunction; however when the portions are brought in proximity to eachother, the activity of the polypeptide is restored. In one aspect,oligomerization of the oligomerization domain brings the portions of thepolypeptide in proximity to each other and restores the function of thepolypeptide. Preferably, oligomerization occurs in response to a signal(e.g., such as the presence of a molecule to which the oligomerizationmolecules must bind in order to oligomerize).

EXAMPLES

The invention will now be further illustrated with reference to thefollowing examples. It will be appreciated that what follows is by wayof example only and that modifications to detail may be made while stillfalling within the scope of the invention.

Example 1 Generating Fusion Molecules by Domain Insertion

A model system consisting of E. coli maltose binding protein (“MBP”) asthe acceptor polypeptide sequence and the penicillin-hydrolyzing enzymeTEM1 β-lactamase as the insertion polypeptide sequence was chosen totest the combinatorial domain insertion strategy for coupling the twoproteins' function. The desired property of the model switch is theability to modulate β-lactamase activity through changes in maltoseconcentration (i.e., the switch molecule or fusion protein would behaveas an allosteric enzyme).

Construction and Testing of Target Plasmid

The E. coli MSB was cloned into plasmid pDIMC8 (Ostermeier and Benkovic,1999, Nat Biotechnol. 17:1205-1209) under control of the IPTG induciblelac promoter to create plasmid pDIMCS-Mal. The MIC for ampicillin ofDH5α/pDIMC8-Mal on LB plates was found to be 30-35 μg/ml.

Construction of β-Lactamase Insert DNA

The β-lactamase gene fragment bla [24-286] (encoding for amino acids24-286 of the β-lactamase gene) was amplified by PGR from pBR322 suchthat it was flanked by EarI restriction enzymes sites. Attempts to clonethis construct into the BamHI site of pACYC184 resulted in very fewtransformants which, upon characterization, were found to containplasmids that lacked the β-lactamase gene fragment. Thus, the firstDNaseI library (described below) was constructed by digesting thebla[24-286] PGR product with EarI. Subsequently, it was found that thebla[24-286] fragment could be cloned into the pTAdv to create the stablevector pTAdv-βlac. Subsequent libraries used a bla[24-286] insertisolated from tins plasmid. It is preferable to use a bla[24-286]fragment derived from a plasmid digest since, unlike the PGR product,the insert DNA will be known not to contain any mutations. However, itmay be useful in the future to create libraries in which the bla[24-286]insert has been mutated by error-prone PGR (see, Caldwell, 1993, supra).Note that the bla[24-286] fragment tor insertion, in this example, doesnot contain a sequence coding for a flexible linker. However, flexiblelinkers can be useful for construction of molecular switches.

Construction of Random Insertion Libraries

Plasmid pDIMC8-Mal was randomly linearized using three differentmethods: (1) DNase/Mn²⁺ digestion followed by polymerase/ligase repair;(2) S1 nuclease digestion followed by polymerase/ligase repair; and (3)S1 nuclease digestion (not repaired). The three protocols differ in (a)the methods used to create the random double-stranded break in thetarget plasmid and (b) whether or not the DNA was repaired bypolymerase/ligase treatment. Digestion was controlled such that asignificant fraction of DNA was undigested in order maximize the amountof linear DNA that only had one double strand break (see, Table 2). Keyfeatures for optimizing the DNase I digestion were the use of Mg²⁺ freeDNaseI (Roche Molecular Biochemicals), a digestion temperature of 22° C.and 1 mM Mn²⁺ instead of Mg²⁺ to increase the ratio of double strandbreaks to nicks (see, e.g., Campbell and Jackson, 1980, supra).

The DNA was repaired using T4 DNA ligase and T4 DNA polymerase (Graf andSchachman, 1996, Proc. Natl. Acad. Sci. USA 93: 11591-11596) (except formethod (3)) and dephosphorylated. Ligation with the bla[24-286] insertDNA and transformation into DB5α produced 10⁵-10⁶ transformants with asmall to large fraction (depending on the method) of the transformantscontaining the bla[24-286] insert (Table 2).

Preparing the Inserted Gene for Insertion

As an example, the preparation of the DNA of the inserted gene will bedescribed for β-lactamase. All the random insertion methods require thatthe inserted DNA (bla) be prepared as a linear piece of dsDNA with bluntends containing only the DNA sequence desired to be inserted. Thedesired DNA is the DNA that codes for amino acids'24 to 286 of TEM-1β-lactamase in pBR322 (bla[24-286]). Amino acids 1-23 are not desiredbecause-they are for signal sequence that targets β-lactamase to theperiplasm. This sequence gets cleaved upon entering the periplasm and isnot part of the mature, active β-lactamase. In the fusion constructs,the natural signal sequence of malE will direct the fusions to theperiplasm. The bla[24-286] DNA will be prepared as in FIG. 2A byamplifying the DNA such that the sequence is between EarI restrictionsites. This DNA is cloned into the BamRI site of pACYC184 to createpACYC-BLA. As shown in FIG. 2A, this construct can be digested with EarIand the bla[24-286] DNA treated with Klenow DNA polymerase to achievethe desired fragment for insertion. This is achieved by virtue of thefact that EarI is a type IIS restriction enzyme that binds anon-palindromic sequence and cleaves outside this sequence.

To achieve the correct geometric configuration and flexibility is thefusions, it may be necessary to include flexible linkers in the fusionsat the insertion site. For example, suitable linkers, include, but arenot limited to: GlyGlyGlySer on the N-terminus and SerGlyGlyGly on theC-terminus. Linkers can be added by amplifying the bla[24-286] DNA suchthat the following DNA sequence 5′-GGTGGTGGCAGC-3′ is added to the 5′end and the sequence 5′-AGCGGTGGCGGC-3′ is added to the 3′ end.

Construction and Characterization of Insertion Libraries

Two general methods are employed: (1) insertion into a plasmid with arandom double-stranded break prepared by nuclease digestion and (2)insertion into a gene using CP-ITCHY.

For the former, three related strategies differing in the nature andorder of use of the nucleases will be used to construct create a single,double strand break in a plasmid containing the MBP: (1) limited DNaseIdigestion in the presence of Mn²⁺, (2) limited DNaseI digestion in thepresence of Mg²⁺ to produce a single nick followed by S1 nuclease ormung bean nuclease digestion to cleave opposite the nick (3) limiteddigestion with S1 nuclease (S1 nuclease can convert supercoiled circularDNA to linear DNA by first making a nick on one of the two strands andthen cutting across from this nick (Germond, et al., 1974, Eur J Biochem43: 591-600), particularly under conditions of low ionic strength(Gonikberg, 1979, Mol. Biol. (Mosk) 13: 1064-9).

Although the first two methods have been used for linker scanningmutagenesis (the random insertion of short sequences), there is littlepublished data on the nature of the sequences at the insertions site ofthe naïve libraries, and this data is sometimes conflicting. Preferably,for all libraries generated, random members of the naïve libraries areselected and the DNA at the insertion sites sequenced to quantify thedistribution and sizes of: deleted DNAS direct insertions and tandemlyduplicated DNA at the insertion site. In particular, insertions in whichsequences of the insertion sequence are tandemly duplicated may beuseful for the same reasons that protein fragments that exhibit proteinfragment complementation often have overlapping sequences. Suchoverlapping sequences are thought to transiently protect exposed regionsduring folding. Duplications or deletions also are likely to beimportant for creating molecular switches by affecting the distance andinteractions between insertion and acceptor sequences.

Incremental truncation methods also can be used for generating librariesof molecules to provide fusion molecules which have larger deletions andtandem duplications at the insertion site. The size of these tandemduplications (or even deletions) can be controlled by size selection ofthe library.

Selection of Active Fusions: β-Lactamase-MBF Fusions

Once β-lactamase-MBP insertion libraries have been constructed, they aresubjected to selection to identity those library members that have bothβ-lactamase and MBP activity as well as those in which β-lactamaseactivity depends on the presence or absence of maltose. The selectionscheme is outlined in FIG. 2D. Fusions with a functional β-lactamasedomain can be identified by growth of bacteria expressing the fusions onplates containing Amp. Fusions whose β-lactamase activity requiresmaltose can be identified by plating bacteria on Amp/maltose plates andthen replica-plating onto Amp plates to identify clones which grow onthe former and do not grow on the latter. Fusions whose β-lactamaseactivity requires the absence if maltose can be identified by platingbacteria on Amp plates and then replica-plating onto Amp/maltose platesto screen for clones which fail to grow on the former and do grow on thelatter.

An alternative screen also is possible. The first screen is carried outas before. On the second screen, the plates will not contain anyampicillin, but still will or will not contain maltose (e.g., the screenis the opposite of the first screen). Filter paper soaked in anitrocefin solution is overlaid on the colonies for a short period oftime. Since nitrocefin is a yellow-colored compound, initially thefilter paper will be uniformly yellow (absorbance peak at 390 nm).However, those library members with β-lactamase activity will degradethe nitrocefin to hydrolyzed nitrocefin which is a red compound(absorbance peak at 485 nm) (O'Caliaghan, et al., 1972, Antimicrob. Ag.Chemother. 1: 283-288). Colonies that fail to turn the filter paper redare identified as those that lack β-lactamase activity under the chosenconditions.

Yet another screen is also possible which relies on the use ofFluorescence Energy Transfer (see, e.g., Zlokarnik, et al., 1998,Science 279: 84-88). For example, the substrate CCF2/AM is not chargedand can cross the membrane of mammalian cells to enter the cytoplasmwhere non-specific esterase remove the ester functionalities of thesubstrate to create CCF2. In CCF2, the cephalosporin core links a7-hydroxycoumarin to fluorescein. In the intact molecule, excitation ofthe coumarin results in FRET to the fluorescein, which emits greenlight. Cleavage of CCF2 by β-lactamase results in spatial separation ofthe two dyes, disrupting FRET such that excitation of the coumarin nowgives rise to blue fluorescence. Charges on CCF2 and its beta-lactmasecleavage products prevent it from leaving the cytoplasm. Thus, FACS andcell sorting can be performed, with and without maltose, to identityfusions in which beta-lactamase activity is dependent on maltose bymonitoring FRET. Generally, any substrate comprising a suitable FRETdonor and acceptor pair can be used to monitor the enzymatic activity offusion molecules according to the invention. The above three methodswill identity ON/OFF switches (i.e., switches in which maltose has avery large effect on β-lactamase activity). In the event that suchON/OFF switches are sufficiently rare or do not occur, and/or toidentify switches in which maltose has a more modest effect, aFRET-based method (e.g., such as based on CCF2) or a spectrophotometricassay can be performed to screen for threshold levels or ranges ofβ-lactamase activity (see, e.g., Baneyx and Georgiou, 1989, EnzymeMicrob. Technol. 11: 559-567; Sigal, et al., 1984, J. Biol. Chem. 259:5327-32). Such an assay can be modified for high throughput screening ofthe activity.

In one aspect, cultures are grown of library members mat exhibitβ-lactamase activity in the malE′ strain PM9F′ (Betton and Hofmung,1994, EMBO J. 13: 1226-1234). When grown, on minimal plates with maltoseas the sole carbon source, cells expressing desired fusions have bothβ-lactamase activity and the ability to bind maltose. Such cells can beexpanded in multi-well plates (e.g., such as microliter plates), lysedusing lysozyme/detergent (e.g., Sambrook, et al., 1989, In MolecularCloning: A Laboratory Manual, Cold Spring Harbor Laboratory, Cold SpringHarbor, N.Y.), and treated wife DNase and RNase. The insoluble fractionis removed by centrifugation and the cleared lysates are assayed in thepresence and absence of maltose for β-lactamase activity by themeasuring a decrease in penicillin G spectrophotometrically at A₂₃₂.Since the goal is to find differences in activity with and withoutmaltose, variations between library members in total fusion proteinproduction, growth of the cells and degree of lysis is not a significantconcern.

Evaluation of the Insertion Libraries

Sequencing was performed on random members of the insertion librariesconstructed using DNaseI or S1 nuclease (see table below). All sequenceswere unique and were distributed throughout the plasmid (supporting therandomness of the methods). Both methods created libraries wife tandemduplications, direct insertions and deletions. The data strongly suggestthat distribution of tandem duplications and deletions in librariescreated by the S1 nuclease method were in a much narrower range.

TABLE 1 Location, Orientation And Nature Of Sequences At Insertion SiteFor DNAse And S1 Nuclease Created Random Domain Insertion Libraries % in% in Deletions (−) MalE “forward” Direct insertions (0) Method genedirection Tandem Duplications (+) DNaseI-repaired 75% 40% +18, +7, +1,+1 library 2 (15/20) (8/20) 0 −5, −13, −16, −17, −42, −48, −54, −56,−75, −162, −191, −263, −340, −379 S1 Nuclease 45% 27% +5, +4 repaired (5/11) (3/11) 0 −1, −1, −2, −2, −5, −6, −22, −101

Roughly 1% of the transformants that had a plasmid wife a bla[24-286]insert, regardless of the method of library construction, could grow on50 μg/ml AMP. Randomly selected Amp^(R) library members were sequenced.All sequences were unique (supporting the ‘randomness’ of insertion) andTable 2 describes whether they contained deletions, tandem duplications,or neither (direct insertion) and whether both fusion points werein-frame or not. Predominantly the Amp^(R) colonies had an N-terminalfragment of the MBP gene fused in frame to bla[24-286] with theremaining fragment of the MBP gene being out of frame. The distributionsin Amp^(R) library members suggest that deletions predominate in theDNase I protocol and feat not repairing plasmid linearized with S1nuclease can bias the library toward direct insertions (though thefraction of library members without an insert increases significantly).In DNaseI library #2, 63% (10/16) of library members in the naïvelibrary comprising the β-lactamase gene had it inserted in the MBP gene.This frequency is higher than that expected based solely on the fractionof DNA in fee plasmid that codes for the MBP gene since insertions atmany locations other than the MBP gene (e.g., Cm^(R) gene, origin ofreplication) do not make viable, Cm^(R) plasmids.

TABLE 2 Comparison of Domain Insertion Libraries Distribution Deletions(−) Of Direct Insertions (0) pDIMC8- Frequency Frequency Tandem FractionIn Mal Of of Duplications (+) In Frame At After Transformants Amp^(R)Randomly Selected Both Method Digestion Transformants With Insert^(a)Colonies Amp^(R) Colonies Crossovers DnaseI 51%  ~5 × 10⁵ ~0.18 0.0017−95, −58, −20,  0/10 repaired supercoiled −10, −5, −3, −1 Library 23%nicked   0 #1 26% linear +1, +51 DnaseI 27% ~10 × 10⁵  ~0.70 0.0079 −15,−11, −10 2/6 repaired supercoiled −8, −5, 0 Library 44% nicked +1 #2 28%linear S1 24% 1.8 × 10⁵ ~0.25 0.0023 −2 0/1 nuclease supercoiledrepaired 42% nicked 34% linear S1 24% 1.0 × 10⁵ ~0.06 0.0005 −2 3/4nuclease supercoiled 0, 0, 0 (not repaired) 42% nicked 34% linear

It is desirable to eliminate members of the library which haveβ-lactamase activity and consist of an N-terminal fragment of malE fusedto an inserted β-lactamase gene with the C-terminal fragment of malEbeing out of frame with the inserted gene to eliminate members of thelibrary incapable of coupling maltose binding to β-lactamase activity.

This can be accomplished in a secondary screen by introducing thelibrary into the auxotrophic strain PM9F′ which contains a deletion ofthe MBP gene, growing the bacteria under conditions such that maltose isthe sole carbon source and selecting for MBP activity as well as forβ-lactamase activity (see, FIG. 2D). Without a functional MBP protein,PM9F′ will not grow. In this way, fusions that have a functional insertand can bind maltose will be identified. Table 3 shows three fusionswith both beta-lactamase activity and the ability to transport maltosein E. coli identified by this method. As can be seen, the selectedfusions consist of both tandem duplications and deletions of the maltosebinding protein at the insertion site. One caveat to this secondaryscreen, however, is that library members that can bind maltose but alterthe ability of MBP to interact correctly with other proteins involved inmaltose transport (e.g., MalF and MalG) will not be selected.

Table 3 summarizes locations of insertions in fusion molecules whichcomprise both β-lactamase and MBP activities.

TABLE 3 Locations Of Insertions Found By Random Insertion With Bothβ-Lactamase And MBP Activities Net Residues Deleted (−) Structure RegionPreviously Sequence Of Or Tandemly Inserted Found To TolerateBifunctional Fusions Duplicated (+) Into Short Insertions?*MBP[1-163]-BLA- −12 Beta sheet yes MBP[174-397] MBP[1-175]-BLA- −5 Betasheet yes MBP[179-397] MBP[1-246]-BLA- +8 Bata sheet No MBP[238-397]*Duplay, et al., 1987., J Mol Biol 194: 663-73.

An analysis of eighteen randomly selected naïve library members of aDNAse-repaired library, generated as described above, was performed todetermine the exact site and orientation of insertions in the library.Thirteen (72%) of the eighteen members of the library included insertionsequences (BLA sequences) inserted at random in the MBP acceptorsequences. The majority of library members (14/18) had deletions ofacceptor sequences at the insertion site, though a direct insertion andthree tandem duplications were also found. Fifty percent of the library(9/18) had deletions and duplications of less than or equal to eighteenbases. Although large deletions are almost certain to be deleterious forfunction, small deletions and tandem duplications are an importantsource of diversity in the library.

From a library of 1.06×10⁶ transformants of the DNAseI library, 0.8%(approximately 8,000 members) could grow on 50 μg/ml LB/AMP platesindicating a functional β-lactamase protein. Sequencing of plasmid DNAfrom random AmpR colonies showed that: library members with anN-terminal fragment of the MBP gene fused in frame to bla[24-286] withthe remaining fragment of the MBP gene being out of frame predominated,this sublibrary. The plasmid DNA from all Amp resistant colonies wasisolated en mass and transformed into the MBP auxotroph PM9F′, a strainunable to grow on minimal media with maltose as a sole carbon sourceunless the MBP is provided in trans (Betton and Hofnung, 1994, EMBO J.13(5): 1226-1234). In the malE auxotroph approximately 10% (i.e., about800 members) of the sublibrary could grouw on a 50 μg/ml AMP minimalplate containing 0.2% maltose, indicating that MBP could transportmaltose in E. coli. Analysis of these bifunctional library membersindicated that the insertions were predominantly localized to threelocations in the MBP protein: near the C-terminus, near residue 170 andnear residue 210. Randomly and non-randomly selected library memberswere sequenced (see, Table 4 below). The sites for successful insertioncorrelate well with results on linker scanning mutagenesis (randominsertion of short DNA sequences) in MBP (see, e.g., Betton, et al.,1993, FEBS Lett. 325 (1-2): 34-8.).

TABLE 4 Locations Of Insertions of β-Lactamase into MBP Where FusionsAre Bifunctional* Sequence of other bifunctional Sequence of functionalMBP Sequence of randomly selected BLA-MBP fusions (not variants found bylinker bifunctional BLA-MBP fusions randomly selected) scanningmutagenesis** Δ134-142 (2) T164-166; Δ164-170; T166-175 Δ163-175; T163(2); T164; Δ162-177 (3) (2); T167; T167-170 (3); E166/167; T166-167;Δ170-171; Δ168-184; T172; T179-184 Δ175-179 T213-220 Δ207-216 (3);Δ212-220 (2); E285/286 (3) E306/307 Δ297-312 (3); Δ301 (2); Δ301-306(3); Δ304-309 (3); Δ304-312 (3) T318 (3) Δ367-368; T369; O362; O367;Δ367-368; T369-370 O370 *Δ means deletion of the indicated MBP residuesat the insertion point of BLA. “T” means a tandem duplication of theindicated MBP sequences at the insertion point. The duplicated residuesare on either side of the BLA sequence. “E” means that insertion of BLAwas exactly between the indicated residues of MBP. “O” (“out of frame”)is the number of the residue of MBP that the N-terminus of BLA is fusedto; the remaining sequence is the out-of-frame sequence that theC-terminus of BLA is fused to. For the BLA-MBP fusion proteins, thenumber in parenthesis is the number of times the sequence was found. Forthe linker scanning mutagenesis, the number in parenthesis is the numberinserted into MBP. *Betton, et al., 1993, FEBS Lett. 325 (1-2): 34-8.

Identification of Switches

In an initial examination of the behavior of these bifunctionalproteins, overnight inoculums of PM9F9 cells bearing nine of thesequenced members of the library were lysed by French press and thesoluble fractionassayed by nitrocefin hydrolysis (O'Callaghan, et al,1972, Antimicrob. Ag. Chemother. 1: 283-288) with and without 50 mMmaltose. One member, T369-370 (i.e., comprising a β-lactamase insertedsuch that amino acids 369 and 370 of MBP were tandemly duplicated oneither side), exhibited an increase in velocity in the presence ofmaltose but not sucrose. Amino acid 370 is the last amino acid of MBP;thus, T369-370 is essentially an end-to-end fusion. Removal of aminoacid residues 369 and 370 from the C-terminus to produce an exactend-to-end fusion (“MBP-BLA”) resulted in a fusion that exhibited astimulation of nitrocefin hydrolysis in the presence of maltose of thesame magnitude as T369-370. It was unexpected that such an end-to-endfusion would result in a switch since end-to-end fusions of MBP and BLAwith linkers have not been reported to behave as switches (see, e.g.,Betton, et al., 1997, Nat. Biotechnology 15: 1276-1279). In addition,the β-lactamase activity of one of the other nine bifunctional proteinstested that has a similar sequence (□367-368) was not modulated bymaltose.

To identify other switches, a semi-rapid throughput assay was developedin which cultures of random bifunctional library members were grown in96-well format in the presence of IPTG, resulting in the accumulation ofthe bifunctional protein in the media. The cultures were centrifuged topellet the cell and the media was assayed spectrophotometrically for thevelocity of β-lactamase hydrolysis of nitrocefin the presence andabsence of 5 mM maltose in a 96-well format. The concentration ofnitrocefin used was the same as the K_(m) for nitrocefin of wild-type8-lactamase so that switches in which maltose binding affected eitherk_(cat) or K_(m) could be identified. Any culture in which there was adifference in rate of more than 20% (between with and without maltose,to eliminate differences due to variability in protein production) wasselected for farther investigation. In a screening of 303 librarymembers, a second library member that showed an increase in velocity ofnitrocefin hydrolysis in the presence of maltose, but not in thepresence of sucrose or glucose, was found three times—T164-165 (i.e.,β-lactamase was inserted such that amino acids 164 and 165 of MBP weretandemly duplicated on either side).

The criteria for bifunctionality in the above screens wasquite-stringent: the fusions were required to have beta-lactamaseactivity and to be able to transport maltose in E. coli. Transportrequires maltose binding, a conformational change in MBP upon maltosebinding, and the requisite interactions with membrane proteins MalG andMalE. Thus, library members that bind maltose but cannot interact withMalG and MalF are not selected (are not bifunctional by definition). Thesites for successful insertion of β-lactamase into MBP to make abifunctional protein correlate quite well with permissive sites in MBPthat tolerate short insertions/deletions (Betton, et al., 1993, FEBSLett. 325(1-2): 34-8) and protein bisection (Betton, et al., 1994, EMBOJ. 13(5): 1226-1234). Thus, the striking observations of thosestudies—that permissive sites were often within α helical and β strandstructural elements—is repeated here. Bifunctional fusion □163-175deletes an entire β-sheet and bifunctional fusion T213-220 tandemlyduplicates two-thirds of an α-helix. Permissive sites tor randominsertions of GPP into the cAMP-dependent protein kinase regulatorysubunit have also included ones within α helices (Biondi, et al., 1998,Nucleic Acids Res. 26(21): 4946-4952).

Two of the five permissive sites for linker scanning mutagenesis andprotein fragment complementation (˜133 and −285) were not observed to bepermissive for domain insertion in this study. However, in a previousstudy, β-lactamase, with 4-5 amino acid linkers on each end, wassuccessfully inserted into MBP at 133 (Betton, et at, 1997, Nat.Biotechnology 15: 1276-1279), suggesting that linkers may be required atthis site. The reason that insertions at 285 were not found could bethat insertions at these locations (a) do not result in folded proteins(b) are not conducive to bβ-lactamase activity or maltose binding or (e)prevent the correct association of MBP with membrane proteins MalG andMalF-an association required for maltose transport. However, with regardto the latter possibility, the sites of interaction between MBP and MalGand MalF (amino acids 13, 14 and 210 which were identified by geneticanalysis (Hor and Shuman, 1993, J. Mol. Biol. 233(4): 659-70) are distalto amino acid 285.

Kinetic Characterization of Switches

In one aspect, the kinetic constants and binding constants of theoriginal wildtype genes, the two switches (T164-165 and MBP-BLA) and twobifunctional non-switches with similar sequences to the switches (T164and □367-368) were determined from Eadie-Hofstee plots and Eadie plotequivalents, respectively, using a spectrophotometric assay fornitrocefin hydrolysis (Sigal, et al., 1984, J. Biol. Chem. 259(8):5327-32). These results of this assay are summarized in Table 5, below,

TABLE 5 Kinetic And Binding Constants Of β-Lactamase-MBP MolecularSwitches^(a) K_(m) nitrocefin (μM) K_(d) maltose 5 mM k_(cat)(+maltose)k_(cat)/K_(m)(+maltose) Sequence (μM) Maltose No maltose k_(cat)(−maltose) k_(cat)/K_(m)(−maltose) β-lactamase +  1-1.5^(c) 47 ± 6 44 ±3 1.0 ± 0.1 1.0 ± 0.2 MBP^(b) T164-165 3.2 ± 1.0 45 ± 4 61 ± 8 1.4 ± 0.11.9 ± 0.3 T369-370 ~10 ~42 ~34 ~1.7 MBP-BLA 14 ± 7  46 ± 3 30 ± 3 1.8 ±0.1 1.2 ± 0.2 ^(a)Conditions: 22° C., 0.1M phosphate (pH 7.0) 1 mM EDTA(+5 mM maltose where indicated); ^(b)β-lactamase and MBP present asseparate proteins; ^(c)Schwartz et. al (Schwartz, Kellermann et al.1976)

Following such a procedure, the Eadie-Hofstee plots for the fusionproteins were linear indicating that the Michaelis-Menten equation holdsfor the switches. The dissociation content of the switches for maltosewas determined using change in velocity of nitrocefin hydrolysis as asignal. The absolute values of k_(cat) are not known since the totalprotein concentration is not known. The relative k_(cat)'s (and also therelative specificity constants) feat compare with and without maltosecan be determined because the enzyme concentration, though unknown, isthe same for both measurements of V_(max). The measurements of K_(m) fornitrocefin observed herein closely match that of a previous study (54.7μM) (see, Raquet, et al., 1994, J. Mol. Biol. 244(5); 625-39).

The end-to-end fusion shows a larger increase in k_(cat) than T164-164did (80% vs. 40%) but this is compensated for by an increase in K_(m)for the end-to-end fusion. T164-165 shows both an increase in k_(cat)and a decrease in K_(m) in the presence of maltose and also shows anincrease of k_(cat)/K_(m) (90%) in the presence of maltose. T164-165 wasalso the most sensitive switch, with a K_(D) for maltose close to thatof the wildtype MBP. All of the above kinetic characterization wasperformed on the media fraction; however, T164-165, in which a His-taghas been added, was b purified by nickel affinity chromatography to highpurity and has been shown to exhibit switching behavior comparable towhat was seen in the media fraction.

Switching Behavior Correlates with a Conformational Change in MBP

Although MBP can bind many other linear maltodextrins, cyclodextrins andreduced or oxidized variants thereof, only those ligands which, induce aconformational change in MBP (Hall, et al. (1997) J. Biol. Chem.272(28): 17605-17609; Ball, et al. (1997) J. Biol. Chem. 272(28):17610-17614) behaved as a switch (see, FIG. 8). Binding ofβ-cyclodextrin (which does not produce a conformational change) wasconfirmed by competition experiments in which maltose's effected onβ-lactamase could be competed away with these sugards. This suggestsconformational change in MBP upon ligand binding as a mechanism tor thecoupling achieved between maltose binding and nitrocefin hydrolysis.

The switches apparently function as monomeric enzymes that derive fromthe covalent linkage of non-interacting, monomeric proteins with theprerequisite binding and catalytic functionalities, respectively.

Example 2 MBP: GFP Fusions

Maltose Binding Protein (MBP) and GFP fission molecules are generatedessentially as described above.

Selection of Active Fusions: GFP-MBP

E. coli cells expressing GFP can be sorted based on fluorescence andother parameters using flow cytometry (Daughtery, et al., 2000, Proc.Natl. Acad. Sci. USA 97: 2029-34). Initially, E. coli cells expressingGFP-MBP fusions library are screened to identify cells with significant:green fluorescence and which grown in the presence of maltose (providedin both in the growth medium and during the sorting process) as well toidentify cells that have significant green fluorescence without maltose(absent in both the growth medium and during the sorting process). Cellsselected are re-cultured and cells are sorted for the absence of, or adecrease in, fluorescence under the opposite condition (e.g., in theabsence of maltose where cells were previously grown in the presence ofmaltose, and in the presence of maltose where cells were previouslygrown in the absence of maltose). Cells selected in this second sortingprocess are plated on LB plates with the level of maltose from the firstsort to confirm that a lack of fluorescence is not due to reasons otherthan the effect of maltose (e.g., such as loss of plasmid, deletion ofthe MBP gene, mutations, etc.).

As in Example 1, secondary screens can be used to eliminate librarymembers in which the insertion sequence and the acceptor sequence areout of frame.

Example 3 Generation of Conditional Heterodimers

As a model system, control over the neomycin resistance protein (Neo)(aminoglycoside phosphotransferase APH(3′)-IIa), by conditionalheterodimerization is engineered. Incremental truncation libraries offragments of Neo are used to identify bisection locations in Neo that donot abolish activity by selection on plates that contain kanamycin.

Design of Overlapping Fragments of Neo

To avoid the possibility of individual fragments of Neo being active ontheir own, the starting fragments for incremental truncation aredesigned such that they lack essential residues for functionalitybecause they are already N-terminally of C-terminally truncated. Theseven classes of APHs have very little general sequence homology(Wright, 1999, Front Biosci. 4: D9-21). However, a sequence alignment ofrepresentative members of each class, combined with the known functionsof residues in APH(3′)-IIIa (Wright and Thompson, 1999, Front Biosci. 4:D9-21) suggest that C-terminal fragment Neo[51-264] will be inactivesince it lacks K50 (equivalent to K44 in APH(3′)-IIIa) and thatN-terminal fragment Neo[1-207] will be inactive since it lacks D208(equivalent to D208 in APH(3′)-IIIa). This is a very conservativeselection of fragments as it is likely that fragments longer than theones chosen will also be inactive on their own.

Incremental truncation libraries of the same overlapping fragments arefused to parallel and antiparallel leucine zippers and are selected onplates containing kanamycin. Preferably, cotransformants are plated onincreasing amounts of kanamycin and plated under different conditions(temperature and IPTG level) to select for heterodimers of Neo thatconfer kanamycin resistance. Plasmid DNA from randomly selected Kan^(R)colonies are isolated and re-transformed separately, and together, toconfirm that the Kan^(R) phenotype requires both vectors. The plasmidDNA is then sequenced to identify the DNA that codes for complementingfragments.

Neo fragments that are functional only when fused to leucine zippers canthus be identified. Fusion molecules whose assembly occur when fused toleucine zippers (e.g., forming functional Neo polypeptides) can besubjected to directed evolution (Arnold, et al., 2001, Trends Biochem.Sci. 26: 100-6) to overcome these shortcomings.

Fragments improved by directed evolution (e.g., pairs of fusionmolecules which display at least 2-fold greater activity, preferably, atleast 5-fold, and more preferably, at least ten-fold activity) are fusedto dimerization domains that require a CID, thereby coupling Neoactivity to the presence or absence of the CID will create Neo activitythat is dependent on the CID. For example, fragments of Neo can be fusedto GyrB and tested to see if kanamycin resistance depends on coumermycinor to FK506-binding protein (FKBF) tested to see if kanamycin resistancedepends on rapamycin. Preferably, fragments whose activities areimproved are sequenced to identify relationships between types ofmutations and increases in activity. In some aspects, fragments whoseactivities are not improved or which are actually diminished also aresequenced.

Construction of Control Vector

The neo gene is amplified from plasmid pSV2-Neo by overlap extension PCR(to s remove an internal NcoI site that creates problems for doing theC-terminal truncation) and cloned into the NdeI/SpeI sites of pDIM-N2 tocreate pDIM-N2-Neo(NcoI⁻).

Construction and Testing of Vectors for Incremental Truncation forProtein Fragment Complementation (No Leucine Zippers)

The DNA coding for fragments Neo[1-207] and Neo [51-264] is amplified byPCR from pDIM-N2-Neo(NcoI⁻) and cloned into the NdeI/BamHI sites ofpDIMN2 and the Bg/II/SpeI sites of pDIMCS. The MIC of kanamycin on DH5αon LB plates is determined to verify that pDIMN2-Neo[1-207] andpDIMC8-Neo[51-264], either separately, or together, do not increase theMIC (i.e., to confirm that these fragments are not active bythemselves).

Determination of the Maximum Rate of Recombination

Recombination between pDIMN2 and pDIMC8 plasmids, even in recA mutants,can reassemble an intact gene (see, e.g., Ostermeier et al., 1999, Proc.Natl. Acad. Sci. USA 96: 3562-3567). Thus, in one aspect, the maximumfrequency of recombination is determined by co-transformingpDIMN2-Neo[1-207] and pDIMC8-Neo[51-264] and plating a large number ofcells on plates containing various amounts of kanamycin to identifyclones in which neomycin activity is restored (e.g., clones in whichrecombination is likely to have occurred). This provides a baseline fordetermining the amount of background in the library (e.g., the likelynumber of false positive results obtained).

Construction and Testing of Incremental Truncation Libraries withoutLeucine Zippers

Individual incremental truncation libraries (˜1×10⁶ each) wereconstructed by a protocol previously described by Ostermeier, et al.,2002, In Protein-Protein Interactions: A Molecular Cloning Manual, E.Golemis. Cold Spring Harbor, N.Y., Cold Spring Harbor Laboratory Press.PCR (with primers outside the truncation region) on random coloniesconfirmed the desired range of truncation. These libraries wereco-transformed into DH5α to create a library of 2.5×10⁶ transformants,an order of magnitude larger than the number of possible combinations(=471²) of truncation lengths of the two libraries. These libraries werethen plated at 22° C. and 37° C. on plates with or without IPTGcontaining 5 or 50 μg/ml kanamycin. The frequency of colonies was not asignificant function of growth temperature or IPTG and averaged 0.00022CPU (5 μg/ml Kan)/CFU (no Kan) and 0.00005 CPU (50 μg/ml Kan)/CFU (noKan). Twenty-seven colonies were analyzed and found to be‘large-plasmid’ recombinants pDIM-N2-Neo(NcoI) contamination. Thus, theNeo gene cannot be fragmented between DNA coding for residues 51 and 207to produce to gene fragments capable of producing enough protein withenough activity to provide kanamycin resistance above background. Inother words, Neo is not amenable to unassisted protein reassembly.

Construction of Incremental Truncation Libraries of Neo Fused toAntiparallel Leucine Zippers

The individual incremental truncation libraries were constructed suchthat fragments of Neo were fused on the truncation side to DNA codingfor antiparallel leucine zippers based on those designed by Ghosh, etal., 2000, J. Am. Chem. Soc. 122: 5658. Three different libraries wereconstructed, varying in the nature of the flexible linker between theleucine zipper and the truncated gene: (a) no linker, (b) GSGG linkerand (c) GSGGGSGG linker. The frequency of Kan^(R) colonies was not asignificant function of IPTG; however, approximately 4-10 fold morecolonies grew at 22° C. than at 37° C. suggesting folding/aggregationproblems in many of the fragments. The frequency of recombination wasfound to be stimulated by the presence of the zipper sequences, thoughthe level of recombination was 2-4 lower than the maximum frequency ofrecombination determined earlier. The frequency of Kan^(R) colonies thatwere not recombinants (‘true positives’; at 37° C. on plates withoutIPTG) are shown in FIG. 7A as a function of kanamycin concentration.Libraries with fragments of Neo fused to parallel leucine zippers alsoresulted in conditional heterodimers with similar sequences, but at asignificantly lower frequency.

Randomly selected true positives were selected and the DNA of thefragments sequenced. The plasmid DNA from these true positives wastransformed to confirm that Kan^(R) only resulted from the presence ofboth plasmids. Thus, the method demonstrates the successful generationof molecular switches that form an active aminoglycosidephosphotransferase IIa (Neo) protein (capable of hydrolyzing theantibiotic kanamycin) only when fused to antiparallel leucine zippers.Upwards of twenty distinct heterodimers whose bisection loci cluster inthree regions (FIG. 7B) have been readily identified through selectionon kanamycin plates even though amenable loci pairs occur at a frequencyof less than 1 for every 2000 possible bisection loci. These fragmentsoften had significant overlap and some loci were proximal to the activesite making it unlikely these loci could have been identified throughrational design.

Although conversion to a conditional heterodimer severely compromisedthe Neo resistance of cells by approximately two orders of magnitude,high level Neo resistance (in one case, up to wildtype levels of ˜500μg/ml) has been restored by one round of random mutagenesis (usingerror-prone PGR under conditions such that approximately one mutationper fragment results) and selection on 10⁶ variants of two differentconditional heterodimers (Neo[1-59]zip/zipNeo[59-264] andNeo[1-91]zip/zipNeo[78-264]). For the case ofNeo[1-59]zip/zipNeo[59-264] the following sets of mutation were found ina random sampling of the improved variants that could grow at ˜500μg/ml: C31R/K175E/V198E, C31R/M120L, N58S/R177S/V198E,C31R/D52Q/D118E/Q155L. The improvement ostensibly resulted from anincrease in the kinetic properties of the conditional heterodimers sincethe two “evolved”, zipperless Neo fragments (Neo fragments withmutations but without leucine zippers) could not provide kanamycinresistance and the expression level of the “unevolved” heterodimers andthe “evolved” heterodimers (both with leucine zippers) were very similaras determined by a quantitative ELISA assay using antibodies againstNeo.

Variations, modifications, and other implementations of what isdescribed herein will occur to those of ordinary skill in the artwithout departing from the spirit and scope of the invention and thefollowing claims.

All patents, patent applications, a publications, referenced herein areIncorporated in their entirety herein.

What is claimed is:
 1. A method for modulating a cellular activity in acell or population of cells, comprising providing a fusion molecule tothe cell or population of cells wherein the fusion molecule is encodedby a nucleic acid sequence and is generated according to the methodcomprising: a) creating an insertion nucleic acid sequence library invitro comprising the steps of: i) obtaining an insertion nucleic acidsequence which encodes a polypeptide that recognizes a signal ofinterest; ii) ligating the insertion nucleic acid sequence underconditions sufficient to circularize the insertion nucleic acidsequence; iii) digesting the insertion nucleic acid sequence of ii)under conditions sufficient to randomly introduce a double-strandedbreak to create the insertion nucleic acid sequence library; b) creatingan acceptor nucleic acid sequence library in vitro comprising the stepsof: i) obtaining an acceptor nucleic acid sequence which encodes apolypeptide that produces a measurable change in a desired state,provided the measurable change in state is not fluorescence; ii)ligating the acceptor nucleic acid sequence under conditions sufficientto circularize an acceptor nucleic acid sequence; iii) digesting theacceptor nucleic acid sequence of ii) under conditions sufficient torandomly introduce a double-stranded break to create an acceptor nucleicacid sequence library; c) ligating the nucleic acids of the libraries ofa) and b) in vitro under sufficient conditions such that an insertionnucleic acid sequence will randomly insert into the randomly introduceddouble strand break in an acceptor nucleic acid sequence, d)transforming a suitable host cell in vitro with the ligated libraries ofc); e) selecting for a transformed host cell in vitro expressing themodulatable fusion polypeptide by exposing the transformed host cell tothe signal of interest and identifying the transformed host cell whichexhibits a measurable change of the desired state; f) isolating thenucleic acid sequence encoding the fusion molecule from the transformedhost cell of e); g) cloning the nucleic acid sequence encoding thefusion molecule into a suitable expression vector; h) transferring thesuitable expression vector of g) to the host cell or population of cellsin vitro or in vivo; and exposing the cell or population of cells to thesignal of interest and changing the state of the acceptor portion of thefusion molecule, thereby modulating the cellular activity of the cell orpopulation of cells.
 2. The method for modulating a cellular activity ofclaim 1, wherein at step a) creating an insertion nucleic acid sequencelibrary comprising the steps of: i) obtaining an insertion nucleic acidsequence which encodes a polypeptide that comprises a state; at step b)iii), generating a duplication, deletion, or substitution, at theinsertion site in the acceptor sequence; and; at step e) selecting for atransformed host cell expressing the modulatable fusion polypeptidewherein insertion couples the state of the insertion sequence to thestate of the acceptor sequence and identifying the transformed host cellwhich exhibits a measurable change of the desired state.
 3. The methodaccording to claim 1, wherein the state of the insertion sequence ismodulated.
 4. The method according to claim 1, wherein the state of theinsertion sequence is modulated in response to a change in the state ofthe acceptor sequence.
 5. The method according to claim 1, wherein thestate of the acceptor sequence is modulated.
 6. The method according toclaim 5, wherein the state of the acceptor sequence is modulated inresponse to a change in the state of the insertion sequence.
 7. Themethod according to claim 1, wherein the fusion molecule comprises a newstate.